Add missing reference

Author: russcam

src/Elasticsearch.Net/Connection/HttpConnection.cs

@@ -3,6 +3,7 @@
 using System.IO.Compression;
 using System.Net;
 using System.Text;
+using System.Threading;
 using System.Threading.Tasks;
 
 // ReSharper disable VirtualMemberNeverOverriden.Global