kselftest:sched: remove duplicate include in cs_prctl_test.c

Clean up the following includecheck warning:

./tools/testing/selftests/sched/cs_prctl_test.c:
Include files sys/types.h and sys/wait.h are included more than
once.

No functional change.

Fixed commit header and log:
Shuah Khan <skhan@linuxfoundation.org>

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Changcheng Deng <deng.changcheng@zte.com.cn>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>