avahi: fix avahi access through dbus

Ensure that the avahi dbus service gets installed if dbus support is
enabled, otherwise avahi access through dbus fails (as clients try
to start the daemon and dbus doesn't know how to do this).

Fixed upstream in 0.6.28 (http://avahi.org/ticket/319

) but this
workaround is simpler/safer for now.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>