Add a missing import.
Add a missing import.

The gitlab backend had some logging calls but never imported the logging
module.

Testing Done:
Tested on production.

Reviewed at https://reviews.reviewboard.org/r/8309/

brennie/reviewboard,sgallagher/reviewboard,sgallagher/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,brennie/reviewboard,chipx86/reviewboard,chipx86/reviewboard,davidt/reviewboard,sgallagher/reviewboard,davidt/reviewboard,chipx86/reviewboard,davidt/reviewboard,reviewboard/reviewboard,brennie/reviewboard,davidt/reviewboard,brennie/reviewboard