Dependency Injection with Ninject and Filter attribute for asp.net mvc

I'm writing a custom Authorization Filter for asp.net mvc 3. I need to inject a userservice into the class but I have no idea how to do this.

public class AuthorizeAttribute : FilterAttribute, IAuthorizationFilter
{
    private IUserService userService;
    private string[] roles;

    public AuthorizeAttribute(params string[] roles)
    {
        this.roles = roles;
    }

    public void OnAuthorization(AuthorizationContext filterContext)
    {
        throw new NotImplementedException();
    }
}

I'm using ninject for dependency injection. I do not want to use a Factory or service locator pattern.

My bindings look like this in the global.acsx:

    internal class SiteModule : NinjectModule
    {
        public override void Load()
        {
            Bind<IUserService>().To<UserService>();
        }
    }