Add domain module to the ecosystem of this module

Problem/Motivation

Each module has an 'ecosystem' page, listing all the other modules flagging the module as their ecosystem.
This page is of big help when looking for useful related modules, or when in need for a solution for a specific use-case.

Even though this module requires the https://www.drupal.org/project/domain module, it does not have it in its ecosystem, therefore it does not show up on https://www.drupal.org/project/domain/ecosystem

Proposed resolution

Add 'domain' module to the ecosystem of this module.