selftests: pidfd: add CONFIG_CHECKPOINT_RESTORE=y to config

kcmp syscall is used in pidfd_getfd_test.c, so add
CONFIG_CHECKPOINT_RESTORE=y to config to ensure kcmp is available.

Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>