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

include/wx/xrc/xmlres.h

@@ -293,10 +293,10 @@ public:
 
 protected:
     // reports input error at position 'context'
-    void ReportError(wxXmlNode *context, const wxString& message);
+    void ReportError(const wxXmlNode *context, const wxString& message);
 
     // override this in derived class to customize errors reporting
-    virtual void DoReportError(const wxString& xrcFile, wxXmlNode *position,
+    virtual void DoReportError(const wxString& xrcFile, const wxXmlNode *position,
                                const wxString& message);
 
     // Scans the resources list for unloaded files and loads them. Also reloads