[master] Make intl a required package

Make intl a required package in OC7 (submitting to master)

This is needed by https://github.com/owncloud/core/pull/7254 for the sorting collation.

Also I think I heard discussions about making that package mandatory anyway.

@DeepDiver1975 @bantu @karlitschek 

Side note: is there a reason why we still make many packages optional like fileinfo, bz2. Would it make sense to make them mandatory to avoid additional cases to support ?