ipv6: remove IN6_ADDR_HSIZE from addrconf.h

IN6_ADDR_HSIZE is private to addrconf.c, move it here to avoid
confusion.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>