net: llmnr_responder: Fix uninitialized use Warning

Fixes following warning:
...
subsys/net/lib/dns/llmnr_responder.c:24:0:
subsys/net/lib/dns/llmnr_responder.c: In function ‘recv_cb’:
include/net/net_pkt.h:1203:9: warning: ‘addr’ may be used uninitialized
in this function [-Wmaybe-uninitialized]
  return net_pkt_append(pkt, len, data, timeout) == len;
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
subsys/net/lib/dns/llmnr_responder.c:306:14: note: ‘addr’ was declared
here
  const u8_t *addr;
              ^~~~
...

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>