package/nss-mdns: fix avahi socket location

In buildroot, the avahi socket is located in /var/run/avahi-daemon,
not in /usr/var/run/avahi-daemon/socket.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>