[package] update avahi to 0.6.28

[package] update avahi to 0.6.28

This updates the avahi package from 0.6.25 to 0.6.28, which, according
to the avahi developers, incorporates bugfixes and fixes minor security
issues.

There seems to be an ongoing problem in certain situations where the
avahi-daemon stops working, e.g. see
http://www.mail-archive.com/avahi@lists.freedesktop.org/msg01512.html

It is not known if 0.6.28 fixes that, but at least we will be working
off the latest version.

Signed-off-by: Mike Brady <mikebrady@eircom.net>
git-svn-id: svn://svn.openwrt.org/openwrt/packages@23595 3c298f89-4303-0410-b956-a3cf2f4a3e73