[packages] avahi: remove inappropriate dependency on dbus in Changeset 32330

[packages] avahi: remove inappropriate dependency on dbus in Changeset 32330

This patch fixes an issue reported by Damiano Albani.
Changeset 32330, while fixing one bug, unfortunately introduces another one -- it makes
avahi-daemon always dependent on the dbus package, causing dbus to be
loaded even when it's not needed.

This patch makes avahi-daemon dependent on dbus only is avahi dbus support has been selected.

Signed-off-by Mike Brady <mikebrady@eircom.net>

git-svn-id: svn://svn.openwrt.org/openwrt/packages@33267 3c298f89-4303-0410-b956-a3cf2f4a3e73