Install the static qtfreetype lib

Modules relying on font support via platformsupport (Qt Wayland) need
the static libqtfreetype.a. Add CONFIG+=installed to get it copied
during make install.

Task-number: QTBUG-50659
Change-Id: Ie6252e7f2dda1dc3da97fee258b9d53b7ca349c0
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>