Add Entity API as a dependency when generating features

In #624018: Exportables and Features support for WYSIWYG 7.x we're adding optional features support without creating a dependency on Entity API. Such features require Entity API, but currently it's not added as a dependency.

We can safely add this dependency as, if it is not needed, Features will filter it out. E.g., when adding a Profile2 profile type, only Profile2 is detected as a dependency since it depends on Entity API.