Restore MediaBrowser.Model.Extensions after #2008

Removing this caused lines 270-271 to fail as the function override did
not exist in the .NET 2.2 framework. Restores functionality.

Emby.Server.Implementations/HttpServer/HttpListenerHost.cs

@@ -15,6 +15,7 @@ using MediaBrowser.Controller;
 using MediaBrowser.Controller.Configuration;
 using MediaBrowser.Controller.Net;
 using MediaBrowser.Model.Events;
+using MediaBrowser.Model.Extensions;
 using MediaBrowser.Model.Serialization;
 using MediaBrowser.Model.Services;
 using Microsoft.AspNetCore.Http;