Make wxGCDC::GetGraphicsContext() const.

There is no reason for this accessor to not be const and it prevents other
methods using it from being const too, so change this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68179 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/dcgraph.h

@@ -38,6 +38,6 @@ public:
     /**
        Retrieves associated wxGraphicsContext
     */
-    wxGraphicsContext* GetGraphicsContext();
+    wxGraphicsContext* GetGraphicsContext() const;
 };