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>

Author: rlubos

subsys/net/lib/mqtt_sn/Kconfig

@@ -66,6 +66,8 @@ config MQTT_SN_KEEPALIVE
 config MQTT_SN_TRANSPORT_UDP
 	bool "UDP transport for MQTT-SN"
 	select NET_SOCKETS
+	select NET_IPV4_IGMP if NET_IPV4
+	select NET_IPV6_MLD if NET_IPV6
 
 config MQTT_SN_LIB_N_RETRY
 	int "Number of times to retry messages"