Unbreak build with libxml >=2.12.0

Index: mapwcs20.cpp
--- mapwcs20.cpp.orig
+++ mapwcs20.cpp
@@ -1446,7 +1446,7 @@ int msWCSParseRequest20(mapObj *map,
 
     /* parse to DOM-Structure and get root element */
     if(doc == NULL) {
-      xmlErrorPtr error = xmlGetLastError();
+      const xmlError * error = xmlGetLastError();
       msSetError(MS_WCSERR, "XML parsing error: %s",
                  "msWCSParseRequest20()", error->message);
       return MS_FAILURE;
