compile error/warning on centos7

[justinpryzby]

/home/pryzbyj/git/sqlsmith/postgres.cc: In member function 'void dut_libpq::connect(std::string&)':
/home/pryzbyj/git/sqlsmith/postgres.cc:318:19: error: 'strlen' was not declared in this scope
if (strlen(errmsg))

[pid 6169] execve("/usr/bin/c++", ["/usr/bin/c++", "-std=gnu++1y", "-I/home/pryzbyj/git/sqlsmith", "-isystem", "/usr/pgsql-11/include", "-isystem", "/usr/pgsql-11/include/server", "-c", "-o", "CMakeFiles/sqlsmith.dir/postgres.cc.o", "/home/pryzbyj/git/sqlsmith/postgres.cc"], [/* 31 vars /]) = 0
Process 6170 attached
[pid 6170] execve("/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/cc1plus", ["/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/cc1plus", "-quiet", "-I", "/home/pryzbyj/git/sqlsmith", "-D_GNU_SOURCE", "-isystem", "/usr/pgsql-11/include", "-isystem", "/usr/pgsql-11/include/server", "/home/pryzbyj/git/sqlsmith/postgres.cc", "-quiet", "-dumpbase", "postgres.cc", "-mtune=generic", "-march=x86-64", "-auxbase-strip", "CMakeFiles/sqlsmith.dir/postgres.cc.o", "-std=gnu++1y", "-o", "/tmp/ccfqm4Y6.s"], [/ 33 vars */]) = 0