CI lint wrongly claims that license file does allow using 'include'

Zendesk ticket (internal link): https://gitlab.zendesk.com/agent/tickets/93486

The above customer found that having a GitLab Starter license and linting a CI file with include falsely gives the following error:

Here's the CI code I tested it with:

include: 
  - 'shared.yml'

job1:
  script: "echo this is from job1"

I reproduced the problem in GitLab EE Starter 10.6.0-ee and on GitLab.com. This problem seems isolated to the linting of the CI file. Actually running the above script works as expected.

/cc @mayra-cabrera