make-ring.py: add missing comma in Ubuntu deps

This patch adds a missing comma in Ubuntu dependencies.

Before that the --dependencies command does not work properly and
there are missing dependencies preventing the client to compile.

Change-Id: Iaf73a42576c815aaa100a5f601480ca471a4da3c
Reviewed-by: Alexandre Lision <alexandre.lision@savoirfairelinux.com>