really override IsCanvasWindow() in wxPanel (this wasn't the case because of a missing const before)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_4_BRANCH@17328 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/panelg.h

@@ -82,7 +82,7 @@ public:
     virtual void InitDialog();
 
 #ifdef __WXUNIVERSAL__
-    virtual bool IsCanvasWindow() { return TRUE; }
+    virtual bool IsCanvasWindow() const { return TRUE; }
     virtual bool ProvidesBackground() { return TRUE; }
 #endif