QmlDesigner: fix not used ": "

Change-Id: I197b3dd46d0835d73c550f216a4d5bb51a8ec9f8
Reviewed-by: Qt CI Patch Build Bot <ci_patchbuild_bot@qt.io>
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/plugins/qmldesigner/designercore/model/documentmessage.cpp

@@ -73,7 +73,7 @@ QString DocumentMessage::toString() const
     }
 
     if (!str.isEmpty())
-        QStringLiteral(": ");
+        str += QStringLiteral(": ");
     str += description();
 
     return str;