Set libnss-mdns PREFERRED_PROVIDER

Set libnss-mdns PREFERRED_PROVIDER

There now could be multiple providers of runtime libnss-mdns with
OE-Core providing one by avahi package and meta-oe providing another
one by mdns package. Set PREFERRED_RPROVIDER to be explit and to
suppress the corresponding message:

| NOTE: Multiple providers are available for runtime libnss-mdns (avahi-l=
ibnss-mdns, mdns)
| Consider defining a PREFERRED_RPROVIDER entry to match libnss-mdns

Signed-off-by: Hugo Villeneuve <hugo@hugovil.com>