libxmlxx.updateScript: correct attribute path

Also freeze the ABI version at 2.0

pkgs/development/libraries/libxmlxx/default.nix

···
 
   passthru = {
     updateScript = gnome.updateScript {
+      attrPath = "libxmlxx";
       packageName = pname;
       versionPolicy = "odd-unstable";
+      freeze = true;
     };
   };