selftests: proc: fix warning: _GNU_SOURCE redefined

[ Upstream commit f3ae6c6e8a3ea49076d826c64e63ea78fbf9db43 ]

Makefile already contains -D_GNU_SOURCE, so we can remove it from the
*.c files.

Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>