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/tbargtk.h

@@ -86,6 +86,7 @@ private:
     void Init();
     void GtkSetStyle();
     GSList* GetRadioGroup(size_t pos);
+    virtual void AddChildGTK(wxWindowGTK* child);
 
     GtkToolbar* m_toolbar;
     GtkTooltips* m_tooltips;