Fix missing import in mediawiki.ui

The .box-sizing() LESS function was moved in:

871875bed4b21e3b17d22046e121bf4e434b804b

And the usage in forms.less wasn't finding it anymore.

Change-Id: I7ae97bd5d4aa113af039182ff39817926bbbf3ed
(cherry picked from commit b38c1a2e374387330a8629128c88597ea35e77e3)