replace m_insertCallback with a virtual function, contrary to the old comments a virtual works just fine

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@55354 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/popupwin.h

@@ -41,6 +41,8 @@ protected:
     virtual void DoMoveWindow(int x, int y, int width, int height);
 
 private:
+    virtual void AddChildGTK(wxWindowGTK* child);
+
 #ifdef __WXUNIVERSAL__
     DECLARE_EVENT_TABLE()
 #endif