QmlDesigner.rewriter: fixing memory leak

The destructor has to be virtual to avoid
a memory leak.

Reviewed-by: Roberto Raggi

src/plugins/qmldesigner/designercore/model/rewriteaction.h

@@ -65,6 +65,7 @@ public:
     virtual RemovePropertyRewriteAction *asRemovePropertyRewriteAction() { return 0; }
     virtual ReparentNodeRewriteAction *asReparentNodeRewriteAction() { return 0; }
     virtual MoveNodeRewriteAction *asMoveNodeRewriteAction() { return 0; }
+    virtual ~RewriteAction() {}
 
 protected:
     RewriteAction()