net: mdns_responder: Use memcpy instead of strncpy for iface name

Following warning is printed if using strncpy(), so use memcpy()
instead. Note that this is false positive as there is no error here but
in order to avoid the warning, change the copy function.

subsys/net/lib/dns/mdns_responder.c:1371:25: warning:
'strncpy' output may be truncated copying 7 bytes from a string
of length 8 [-Wstringop-truncation]
 1468 | strncpy(if_req.ifr_name, name, sizeof(if_req.ifr_name) - 1);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

subsys/net/lib/dns/mdns_responder.c:1468:25: warning:
'strncpy' output may be truncated copying 7 bytes from a string
of length 8 [-Wstringop-truncation]
 1468 | strncpy(if_req.ifr_name, name, sizeof(if_req.ifr_name) - 1);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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