netlink: Don't define USE_GNU
Applications should never #define USE_GNU themselves, but rather
_GNU_SOURCE. This patch removes USE_GNU and replaces it with including
config.h which has _GNU_SOURCE defined if applicable for that platform
See for example:
https://gcc.gnu.org/ml/fortran/2005-10/msg00365.html
Reviewed-by: Fabiano FidĂȘncio <fidencio@redhat.com>