tipc: fix build issue when building without IPv6

We can't directly call ipv6_sock_mc_join() but should use the stub
instead and protect it around IS_ENABLED.

Fixes: d0f91938

 ("tipc: add ip/udp media type")
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>