/usr/include/dns_sd.h is in libavahi-compat-libdnssd-dev 0.7-3.1ubuntu1.2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/include/avahi-compat-libdns_sd/dns_sd.h
/usr/include/avahi-compat-libdns_sd/dns_sd.h