Set optional flag to GerritTriggerBuildChooser extension.

Git plugin is optional, So any loading error should not be logged.

Fix for JENKINS-22813: Warning when clean install and restart 
https://issues.jenkins-ci.org/browse/JENKINS-22813
Merge pull request #148 from rinrinne/optional-dependency

Set optional flag to GerritTriggerBuildChooser extension