selftests/user_events: Reenable build

The user_events selftests were removed from the standard set of
selftests due to the uapi header it relies on having been temporarily
removed.  That header is now reinstated so we can reenable the tests.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>