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>