QmlDesigner: Fix warning in WidgetRegistrationInterface

Change-Id: I921f27f788de1d6601cde9cf6301b4fda1c3f0f0
Reviewed-by: Ali Kianian <ali.kianian@qt.io>

src/plugins/qmldesigner/libs/designercore/include/widgetregistration.h

@@ -12,6 +12,9 @@ class WidgetRegistrationInterface
 public:
     virtual void registerWidgetInfo(WidgetInfo) = 0;
     virtual void deregisterWidgetInfo(WidgetInfo) = 0;
+
+protected:
+    ~WidgetRegistrationInterface() = default;
 };
 
 } // namespace QmlDesigner