Make wxXmlResource::ReportError() wxXmlNode parameter const.

This function (and the related DoReportError()) doesn't need to modify its
"context" argument so take a const-pointer in it.

See #11431.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66058 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/xrc/xmlres.h

@@ -388,7 +388,7 @@ protected:
 
         @see wxXmlResourceHandler::ReportError(), DoReportError()
      */
-    void ReportError(wxXmlNode *context, const wxString& message);
+    void ReportError(const wxXmlNode *context, const wxString& message);
 
     /**
         Implementation of XRC resources errors reporting.
@@ -416,7 +416,7 @@ protected:
 
         @see ReportError()
     */
-    virtual void DoReportError(const wxString& xrcFile, wxXmlNode *position,
+    virtual void DoReportError(const wxString& xrcFile, const wxXmlNode *position,
                                const wxString& message);
 };