Fix:

error: invalid conversion from 'const xmlError*' to 'xmlError*'

https://github.com/cegui/cegui/issues/1309
https://github.com/cegui/cegui/commit/285ba5a2c0c435ed865c6e86afe314a822fd1e3f
--- cegui/src/XMLParserModules/LibxmlParser/CEGUILibxmlParser.cpp.orig	2013-01-12 11:07:24.000000000 -0600
+++ cegui/src/XMLParserModules/LibxmlParser/CEGUILibxmlParser.cpp	2024-08-11 11:37:00.000000000 -0500
@@ -105,7 +105,11 @@
 
     if (!doc)
     {
+#if LIBXML_VERSION >= 21200
+        const xmlError* err = xmlGetLastError();
+#else
         xmlError* err = xmlGetLastError();
+#endif
 
         CEGUI_THROW(GenericException(
             String("LibxmlParser::parseXMLFile - xmlParseMemory failed in file: '") +