Revert "selftests/cgroup: Drop define _GNU_SOURCE"

This reverts commit c1457d9a.

The framework change to add D_GNU_SOURCE to KHDR_INCLUDES
to Makefile, lib.mk, and kselftest_harness.h is reverted
as it is causing build failures and warnings.

Revert this change as this change depends on the framework
change.
Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/cgroup/cgroup_util.c

 /* SPDX-License-Identifier: GPL-2.0 */
+
+#define _GNU_SOURCE
+
 #include <errno.h>
 #include <fcntl.h>
 #include <linux/limits.h>

tools/testing/selftests/cgroup/test_core.c

 /* SPDX-License-Identifier: GPL-2.0 */
+
+#define _GNU_SOURCE
 #include <linux/limits.h>
 #include <linux/sched.h>
 #include <sys/types.h>

tools/testing/selftests/cgroup/test_cpu.c

 // SPDX-License-Identifier: GPL-2.0
+
+#define _GNU_SOURCE
 #include <linux/limits.h>
 #include <sys/sysinfo.h>
 #include <sys/wait.h>

tools/testing/selftests/cgroup/test_hugetlb_memcg.c

 // SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+
 #include <linux/limits.h>
 #include <sys/mman.h>
 #include <stdio.h>

tools/testing/selftests/cgroup/test_kmem.c

 // SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+
 #include <linux/limits.h>
 #include <fcntl.h>
 #include <stdio.h>

tools/testing/selftests/cgroup/test_memcontrol.c

 /* SPDX-License-Identifier: GPL-2.0 */
+#define _GNU_SOURCE
+
 #include <linux/limits.h>
 #include <linux/oom.h>
 #include <fcntl.h>

tools/testing/selftests/cgroup/test_zswap.c

 // SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+
 #include <linux/limits.h>
 #include <unistd.h>
 #include <stdio.h>