selftests: fix warning: "_GNU_SOURCE" redefined

Makefile contains -D_GNU_SOURCE. remove define "_GNU_SOURCE"
in c files.
Signed-off-by: NPeng Hao <peng.hao2@zte.com.cn>
Signed-off-by: NShuah Khan (Samsung OSG) <shuah@kernel.org>

tools/testing/selftests/proc/fd-001-lookup.c

@@ -14,7 +14,7 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 // Test /proc/*/fd lookup.
-#define _GNU_SOURCE
+
 #undef NDEBUG
 #include <assert.h>
 #include <dirent.h>

tools/testing/selftests/proc/fd-003-kthread.c

@@ -14,7 +14,7 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 // Test that /proc/$KERNEL_THREAD/fd/ is empty.
-#define _GNU_SOURCE
+
 #undef NDEBUG
 #include <sys/syscall.h>
 #include <assert.h>