Add a missing using statement

Author: michaelklishin

projects/RabbitMQ.Client/client/impl/BasicPublishBatch.cs

@@ -38,6 +38,7 @@
 //  Copyright (c) 2007-2020 VMware, Inc.  All rights reserved.
 //---------------------------------------------------------------------------
 
+using System;
 using System.Collections.Generic;
 
 using RabbitMQ.Client.Framing.Impl;