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>