[BUGFIX] Add t3skin to class loading info in composer.json

With #65528 t3skin was made a protected extension, but it was forgotten
to add the extension to the class loading information array in composer.json

Do this now, so that composer class loading is active
for all protected core extensions.

Releases: master
Resolves: #67207
Change-Id: I9138d84f715c4e372763a5f17ef40f2a176cae10
Reviewed-on: http://review.typo3.org/39817


Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>