net: dns: Join mDNS multicast group for resolving

If mDNS resolver is enabled but mDNS responder is not, then
mDNS multicast address group is not joined. This would prevent
the mDNS resolver to receive the responses. Fix this by
joining the mDNS multicast group if mDNS responder is not
enabled (because the responder will join the group itself).

Fixes #86477

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>