Mk/Uses/qt.mk: Fix metaports after USE_QT change

devel/qt5 and devel/qt6 use a generated USE_QT that wasn't caught by
commit ddae4e92d8313a1461145e3b4f0232fe463feaee:
Mk/Uses: always use colon for build/run suffix

Fix it manually.

Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D36524