selftests: thermal: intel: workload_hint: add missing gitignore

The 'workload_hint_test' test generates an object with the same name,
but there is no .gitignore file in the directory to add the object as
stated in the selftest documentation.

Add the missing .gitignore file and include 'workload_hint_test'.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>