debian/control-in: no more disable glusterfs in Ubuntu (LP: #1246924)

Christian Ehrhardtrequested to merge
paelzer-guest/qemu:glusterfs-ubuntu into master

After years of not working out: https://bugs.launchpad.net/bugs/1274247 we made it in https://bugs.launchpad.net/ubuntu/+source/glusterfs/+bug/1950321 and this can now be enabled on our side as well.

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