Update Startup.cs

Tests/MQTTnet.TestApp.AspNetCore2/Startup.cs

@@ -5,6 +5,7 @@ using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.FileProviders;
+using MQTTnet;
 using MQTTnet.AspNetCore;
 using MQTTnet.Server;