chore: import namespace

Author: MaggieKimani1

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

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