Add using

Author: HaoK

src/Identity/src/Identity/ISecurityStampValidator.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System.Threading.Tasks;
+using Microsoft.AspNetCore.Authentication;
 using Microsoft.AspNetCore.Authentication.Cookies;
 
 namespace Microsoft.AspNetCore.Identity