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/dataview.h

@@ -431,6 +431,7 @@ private:
     wxDataViewCtrlInternal  *m_internal;
     wxDataViewColumnList     m_cols;
 
+    virtual void AddChildGTK(wxWindowGTK* child);
     void GtkEnableSelectionEvents();
     void GtkDisableSelectionEvents();