Ignore config/Gitlab.gitlab-license file

What does this MR do?

Ignore config/Gitlab.gitlab-license file.

ee/lib/tasks/gitlab/license.rake tries to load Settings.source.dirname + 'Gitlab.gitlab-license' which expands to config/Gitlab.gitlab-license for a default GDK installation.

This file should be ignored so developers can put their license files in config directory and not worry about committing the file.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done