selftests/cgroup: Drop define _GNU_SOURCE

_GNU_SOURCE is provided by lib.mk, so it should be dropped to prevent
redefinition warnings.
Signed-off-by: Edward Liaw <edliaw@google.com>
Signed-off-by: Tejun Heo <tj@kernel.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>