- adds missing using

Author: baywet

src/Microsoft.OpenApi.Readers/V2/OpenApiResponseDeserializer.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
+using System;
 using System.Collections.Generic;
 using Microsoft.OpenApi.Extensions;
 using Microsoft.OpenApi.Models;