get rid of warning about non-virtual dtor with virtual methods.

widget/src/mac/nsWidgetFactory.cpp

@@ -136,7 +136,7 @@ public:
     NS_IMETHOD LockFactory(PRBool aLock);
 
     nsWidgetFactory(const nsCID &aClass);
-    ~nsWidgetFactory();
+    virtual ~nsWidgetFactory();
 private:
   nsCID mClassID;