scanner: allow referencing foreign enums

It's already possible to reference foreign interfaces, so it
should also be possible to reference foreign enums.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Silvan Jegen <s.jegen@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>