Fix build break in Never tests

Ix.NET/Source/System.Interactive.Async.Tests/System/Linq/Operators/Never.cs

@@ -1,7 +1,8 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the Apache 2.0 License.
 // See the LICENSE file in the project root for more information. 
 
+using System;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;