Fix build

Emby.Server.Implementations/SocketSharp/WebSocketSharpRequest.cs

@@ -2,6 +2,7 @@ using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Net;
+using System.Net.Mime;
 using MediaBrowser.Common.Net;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Http.Extensions;