net: mqtt_sn: Make sure multicast functionalities are enabled

UDP transport for the MQTT SN libraries depends on IGMP/MLD APIs
unconditionally (via respective setsockopt calls) and without them being
enabled transport initialization would fail. Therefore, ensure
respective multicast libraries are always enabled if MQTT SN UDP
transport is used.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>