selftests: proc: fix warning: _GNU_SOURCE redefined

Makefile already contains -D_GNU_SOURCE, so we can remove it from the
*.c files.

Signed-off-by: Tommi Rantala <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/proc/proc-loadavg-001.c

···
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 /* Test that /proc/loadavg correctly reports last pid in pid namespace. */
-#define _GNU_SOURCE
 #include <errno.h>
 #include <sched.h>
 #include <sys/types.h>

tools/testing/selftests/proc/proc-self-syscall.c

···
  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
-#define _GNU_SOURCE
 #include <unistd.h>
 #include <sys/syscall.h>
 #include <sys/types.h>

tools/testing/selftests/proc/proc-uptime-002.c

···
  */
 // Test that values in /proc/uptime increment monotonically
 // while shifting across CPUs.
-#define _GNU_SOURCE
 #undef NDEBUG
 #include <assert.h>
 #include <unistd.h>