Don't remove const-ness using C-style cast

Avoid another -Wcast-qual from gcc.

include/wx/motif/statbmp.h

@@ -52,7 +52,7 @@ public:
     wxIcon GetIcon() const
     {
         // don't use wxDynamicCast, icons and bitmaps are really the same thing
-        return *(wxIcon*)&m_messageBitmap;
+        return *(const wxIcon*)&m_messageBitmap;
     }
 
     // for compatibility with wxMSW