Delete double define _GNU_SOURCE

It's already defined in src.c

Author: lucic71

rules/cstring/src.cpp

@@ -1,7 +1,6 @@
 // Copyright (c) 2022-present INESC-ID.
 // Distributed under the MIT license that can be found in the LICENSE file.
 
-#define _GNU_SOURCE
 #include <string.h>
 
 const char *f6(const char *a0, int a1) { return strchr(a0, a1); }