Add copyright header to TransportSocketOptions.cs.

src/Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets/SocketTransportOptions.cs

@@ -1,4 +1,7 @@
-using System;
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
 using System.Collections.Generic;
 using System.Text;