Add usings needed for quickstart so that users don't have to add them themselves

Author: daniv-msft

samples/dotnetcore/getting-started/webfrontend/Controllers/HomeController.cs

@@ -1,7 +1,10 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
+using System;
+using System.Collections.Generic;
 using System.Diagnostics;
+using System.Threading.Tasks;
 using Microsoft.AspNetCore.Mvc;
 using webfrontend.Models;