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 Fidêncio <fidencio@redhat.com>