add using

Author: gavinbarron

src/Microsoft.OpenApi/Validations/Rules/OpenApiSchemaRules.cs

@@ -4,6 +4,7 @@
 using Microsoft.OpenApi.Models;
 using Microsoft.OpenApi.Properties;
 using System.Collections.Generic;
+using System.Linq;
 using System.Runtime.CompilerServices;
 
 namespace Microsoft.OpenApi.Validations.Rules