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 ddae4e92:
	Mk/Uses: always use colon for build/run suffix

Fix it manually.

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