galera: correction after wsrep-lib update

Correction to ensure compatibility with the updated wsrep-lib library.

sql/wsrep_client_service.h

@@ -57,6 +57,10 @@ class Wsrep_client_service : public wsrep::client_service
   {
     return false;
   }
+  bool is_prepared_xa()
+  {
+    return false;
+  }
   bool is_xa_rollback()
   {
     return false;