project.conf: Add git_gnome_org alias
This fixes fetch and track operations which otherwise fail with this kind of error:
/usr/bin/git clone --mirror -n git_gnome_org:yelp-xsl /home/sam/cache/buildstream/sources/git/tmpg5ou7qg4
Cloning into bare repository '/home/sam/cache/buildstream/sources/git/tmpg5ou7qg4'...
ssh: Could not resolve hostname git_gnome_org: No address associated with hostname
fatal: Could not read from remote repository.
Please make sure you have the correct access rights
and the repository exists.