During `.gitlab-ci.yml` configuration file validation there are two
cases:
1- either we are validating a file within a project context (a project
and a sha is given in option parameters)
2- either we are validating a file with no project context (as in the
api POST /ci/lint)

In the first case, all `include:` keywords are replaced by the content
of the included CI cnfiguration files.

In the second case, the `include:` configuration is kept untouched.

The current problem arises in the second case when one wants to
validate a generic gitlab-ci configuration file with an `include:`
keyword from the API. The following error will be raised by the API:
`"jobs:include config should be a hash"`.

This commit fixes the behavior by validating the `include:` keyword if
its present to not parse it as a job definition.

Fixes #55863 / #52822