don't build distancefieldgenerator unless qtquick is present

Fixes: QTBUG-70489
Change-Id: Idcc4c8b2cdb9db6fb28758f56ad80c540518d34c
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/src.pro

@@ -8,7 +8,7 @@ qtHaveModule(widgets) {
                   pixeltool \
                   designer
 
-        qtConfig(thread): SUBDIRS += distancefieldgenerator
+        qtHaveModule(quick):qtConfig(thread): SUBDIRS += distancefieldgenerator
 
         linguist.depends = designer
     }