Unbreak build with libxml >=2.12.0

Index: mapwcs.cpp
--- mapwcs.cpp.orig
+++ mapwcs.cpp
@@ -362,7 +362,7 @@ static int msWCSParseRequest(cgiRequestObj *request, w
     /* parse to DOM-Structure and get root element */
     if((doc = xmlParseMemory(request->postrequest, strlen(request->postrequest)))
         == NULL) {
-      xmlErrorPtr error = xmlGetLastError();
+      const xmlError * error = xmlGetLastError();
       msSetError(MS_WCSERR, "XML parsing error: %s",
                  "msWCSParseRequest()", error->message);
       return MS_FAILURE;
