- adds missing using for linq

Author: baywet

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

@@ -2,6 +2,7 @@
 // Licensed under the MIT license. 
 
 using System.Collections.Generic;
+using System.Linq;
 using Microsoft.OpenApi.Extensions;
 using Microsoft.OpenApi.Models;
 using Microsoft.OpenApi.Readers.ParseNodes;