libshairplay: needs avahi with libdnssd compat option at runtime

libshairplay uses dlopen to open libdnssd at runtime (unless dlopen() isn't
available, then static linking is used - But avahi depends on
!BR2_PREFER_STATIC_LIB so that isn't possible), so ensure avahi is available.

Fixes http://autobuild.buildroot.net/results/54d/54de48b23439cef4cbea3b8a65b20e2a8b049f67/



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>