selftests: vDSO: Explicitly include sched.h

The previous commit introduced the use of CLONE_NEWTIME without including
<sched.h> which contains its definition.

Add an explicit include of <sched.h> to ensure that CLONE_NEWTIME
is correctly defined before it is used.

Fixes: 2aec9003 ("selftests: vDSO: ensure vgetrandom works in a time namespace")
Signed-off-by: Yu Liao <liaoyu15@huawei.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>