add prerequisite libbpf in README

[apowerfulmei]

When I was building a BRF environment from scratch on Docker, I realized that because of the lack of libbpf, it would cause some errors when I try to make the BRF source code. So I think it would be really helpful to add a description of libbpf in the README file.

[apowerfulmei]

Detailed error message is as blow:

In file included from executor/common_linux.h:5691,
                 from executor/common.h:505,
                 from executor/executor.cc:196:
executor/common_brf_linux.h: In function ‘bool check_attach_res(bpf_program*, int*)’:
executor/common_brf_linux.h:96:21: error: ‘bpf_map_create_opts’ was not declared in this scope; did you mean ‘bpf_link_create_opts’?
   96 |         LIBBPF_OPTS(bpf_map_create_opts, opts);
      |                     ^~~~~~~~~~~~~~~~~~~
      |                     bpf_link_create_opts
executor/common_brf_linux.h:96:42: error: ‘opts’ was not declared in this scope
   96 |         LIBBPF_OPTS(bpf_map_create_opts, opts);
      |                                          ^~~~
executor/common_brf_linux.h:96:9: error: ‘LIBBPF_OPTS’ was not declared in this scope; did you mean ‘LIBBPF_API’?
   96 |         LIBBPF_OPTS(bpf_map_create_opts, opts);
      |         ^~~~~~~~~~~
      |         LIBBPF_API
executor/common_brf_linux.h:108:17: error: ‘bpf_program__type’ was not declared in this scope; did you mean ‘bpf_program__title’?
  108 |         switch (bpf_program__type(prog)) {
      |                 ^~~~~~~~~~~~~~~~~
      |                 bpf_program__title
executor/common_brf_linux.h:130:24: error: ‘bpf_map_create’ was not declared in this scope; did you mean ‘bpf_map_freeze’?
  130 |                 *res = bpf_map_create(BPF_MAP_TYPE_SOCKMAP, "brf",
      |                        ^~~~~~~~~~~~~~
      |                        bpf_map_freeze
executor/common_brf_linux.h: In function ‘int bpf_program_attach(bpf_program*, int, bpf_link**)’:
executor/common_brf_linux.h:153:29: error: aggregate ‘bpf_program_attach(bpf_program*, int, bpf_link**)::bpf_tcx_opts tcx_opts’ has incomplete type and cannot be defined
  153 |         struct bpf_tcx_opts tcx_opts;
      |                             ^~~~~~~~
executor/common_brf_linux.h:154:35: error: aggregate ‘bpf_program_attach(bpf_program*, int, bpf_link**)::bpf_netfilter_opts nf_opts’ has incomplete type and cannot be defined
  154 |         struct bpf_netfilter_opts nf_opts;
      |                                   ^~~~~~~
executor/common_brf_linux.h:160:48: error: ‘bpf_program__type’ was not declared in this scope; did you mean ‘bpf_program__title’?
  160 |                       libbpf_bpf_prog_type_str(bpf_program__type(prog)));
      |                                                ^~~~~~~~~~~~~~~~~
      |                                                bpf_program__title
executor/common_brf_linux.h:160:23: error: ‘libbpf_bpf_prog_type_str’ was not declared in this scope
  160 |                       libbpf_bpf_prog_type_str(bpf_program__type(prog)));
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~
executor/common_brf_linux.h:166:17: error: ‘bpf_program__type’ was not declared in this scope; did you mean ‘bpf_program__title’?
  166 |         switch (bpf_program__type(prog)) {
      |                 ^~~~~~~~~~~~~~~~~
      |                 bpf_program__title
executor/common_brf_linux.h:178:25: error: ‘bpf_program__attach_tcx’ was not declared in this scope; did you mean ‘bpf_program__attach_xdp’?
  178 |                 *link = bpf_program__attach_tcx(prog, res, &tcx_opts);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~
      |                         bpf_program__attach_xdp
executor/common_brf_linux.h:224:14: error: ‘BPF_PROG_TYPE_NETFILTER’ was not declared in this scope; did you mean ‘BPF_PROG_TYPE_SOCKET_FILTER’?
  224 |         case BPF_PROG_TYPE_NETFILTER:
      |              ^~~~~~~~~~~~~~~~~~~~~~~
      |              BPF_PROG_TYPE_SOCKET_FILTER
executor/common_brf_linux.h:225:25: error: ‘bpf_program__attach_netfilter’ was not declared in this scope; did you mean ‘bpf_program__attach_iter’?
  225 |                 *link = bpf_program__attach_netfilter(prog, &nf_opts);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                         bpf_program__attach_iter
At global scope:
cc1plus: note: unrecognized command-line option ‘-Wno-unused-command-line-argument’ may have been intended to silence earlier diagnostics
make: *** [Makefile:143：executor]

#46 encountered the same issue, I guess it solved this issue by changing the library search path to where the libbpf is installed in Makefile

	$(CC) -o ./bin/$(TARGETOS)_$(TARGETARCH)/syz-executor$(EXE) executor/executor.cc \
		$(ADDCFLAGS) $(CFLAGS) -L/usr/local/lib64 -lbpf -lelf -lz -DGOOS_$(TARGETOS)=1 -DGOARCH_$(TARGETARCH)=1 \
		-DHOSTGOOS_$(HOSTOS)=1 -DGIT_REVISION=\"$(REV)\"

change -L/usr/local/lib64 to -L/usr/lib64