libostree: make sure libavahi-client is built

Avahi support also needs libavahi-client, which is built by avahi if avahi-daemon
and dbus is selected.

Fixes:
http://autobuild.buildroot.net/results/1edfb60add17945eabc557e1dbfb08a0ba466df9/



Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Tested-by: Matt Weber <matthew.weber@rockwellcollins.com>
[Thomas: drop dependency on dbus, since libostree only needs avahi.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>