selftests/user_events: add a note about user_events.h dependency

This test depends on <linux/user_events.h> exported in uapi
The following commit removed user_events.h out of uapi:
commit 5cfff569 ("tracing: Move user_events.h temporarily out
   of include/uapi")

This test will not compile until user_events.h is added back to uapi.

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>