package/shairport-sync: fix avahi dependency

shairport-sync uses mDNS to pubish its service. This task is implemented
(among others) in avahi and tinysvcmdns.

To use avahi as the mDNS backend, shairport-sync requires libavahi-client or
libdns_sd. Both will work, but libavahi-client is sufficient.

To get libavahi-client support from avahi BR2_PACKAGE_AVAHI_DAEMON and
BR2_PACKAGE_DBUS needs to be selected. Unfortunatly this is not immediately
obvious if you've not checked avahis configure file. A
BR2_PACKAGE_LIBAVAHI_CLIENT config symbol may help here for clarification,
but is not present yet.

Fixes autobuild errors:
  - http://autobuild.buildroot.net/results/a6a/a6ab7641a3cafcdaeffe11f08d5fcacbbb882d8d/
  - http://autobuild.buildroot.net/results/cb4/cb41980f2fb416d627bea0950e1dae727eae0ca6/
  - http://autobuild.buildroot.net/results/009/00929df01474813be576936e80e86374efdbc327/
  - http://autobuild.buildroot.net/results/a40/a40b8cf9aaa4ade4d96524219276cfad2920104c/
  - http://autobuild.buildroot.net/results/03f/03fff2805b3348f2557bbef73a716eadea3add03/



and some more.

Signed-off-by: Jörg Krause <jkrause@posteo.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>