net: ipv6: Make Multicast Listener Discovery API public

IPv6 MLD API was so far defined in an internal header. This does not
seem correct though, as application code should be able to join/leave
multicast groups, hence the API should be exposed in a public header,
just as it is done for its IPv4 countepart - IGMP.

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