You don’t need to reference ASP.NET MVC. You can use the StopRoutingHandler which implements IRouteHandler like so:
routes.Add(new Route("{resource}.axd/{*pathInfo}", new StopRoutingHandler()));
This is part of .NET 3.5 SP1 and doesn’t require MVC. The IgnoreRoutes method is a convenience extension method which is part of ASP.NET MVC.