selftests/user_events: add gitignore file

user_events builds a series of binaries that can be ignored by git.
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/user_events/.gitignore

+abi_test
+dyn_test
+ftrace_test
+perf_test