Billings menu item needs an icon on the new navigation bar

Billings menu need some with a menu icon:

cc @winh @hazelyang

Thanks @oswaldo! I think we already created an icon for Billings in https://gitlab.com/gitlab-org/gitlab-ee/issues/3297 , so I'll close this issue.

closed

This was resolved with https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2814.