selftests: fix warning: "_GNU_SOURCE" redefined

[ Upstream commit 0387662d1b6c5ad2950d8e94d5e380af3f15c05c ]

Makefile contains -D_GNU_SOURCE. remove define "_GNU_SOURCE"
in c files.

Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>