net: mdns: Fix unused variable warnings

Fix GCC "warning: 'ipv4' defined but not used [-Wunused-variable]" and
"warning: 'ipv6' defined but not used [-Wunused-variable]" when either
IPv4 or IPv6 support is not enabled.

Signed-off-by: Markus Fuchs <markus.fuchs@de.sauter-bc.com>