net/ipv6: Enforce MLD to be selected if ND is enabled

As noted in net_if.c:net_if_ipv6_addr_add() IPv6 ND needs MLD.

It is not selected during test, as the combination of ND without DAD/MLD
breaks (something to study/fix further it seems).

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>

subsys/net/ip/Kconfig.ipv6

@@ -143,6 +143,7 @@ config NET_IPV6_NBR_CACHE
 config NET_IPV6_ND
  bool "Activate neighbor discovery"
  depends on NET_IPV6_NBR_CACHE
+ select NET_IPV6_MLD if !NET_TEST
  default y
  help
  The value depends on your network needs. ND should normally