abstractions/X: Allow (only) reading X compose cache

Merged Christian Boltz requested to merge cboltz/apparmor:cboltz-x-compose-cache into master

... (/var/cache/libx11/compose/*), and deny any write attempts

Reported by darix, https://git.nordisch.org/darix/apparmor-profiles-nordisch/-/blob/master/apparmor.d/teams