Fix the native header column minimal width determination

wxGrid::AutoSizeColumn can set per column minimal width and this values
must be used for native headers columns.

include/wx/generic/grid.h

@@ -2282,6 +2282,7 @@ protected:
     friend class wxGridWindow;
     friend class wxGridHeaderRenderer;
 
+    friend class wxGridHeaderColumn;
     friend class wxGridHeaderCtrl;
 
 private: