xwayland: comment on cleanup_after_cairo()

I tried this, and it causes a crash. Leave a note for the future when we
happen to use some other backend with xwayland and find a "leak".

The reason this is a comment and not a Gitlab issue is that you probably
would not go looking for a Gitlab issue saying an idea is a bad one.
This comment is more likely to be found.

It's not really a leak either, it only needs to be fixed if you want a
clean ASan leak report.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>