net: mdns: Fix unused variable warnings
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 <73cc590cffe31834ed0e72b5f2040b6b5f73acae@de.sauter-bc.com>

Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr