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
and not worry about commiting the file.

.gitignore

@@ -34,6 +34,7 @@ eslint-report.html
 /config/database*.yml
 /config/gitlab.yml
 /config/gitlab_ci.yml
+/config/Gitlab.gitlab-license
 /config/initializers/rack_attack.rb
 /config/initializers/smtp_settings.rb
 /config/initializers/relative_url.rb