ipv6: fix IPV6_PKTINFO with v4 mapped

Use IS_ENABLED(CONFIG_IPV6), to enable this code if IPv6 is
a module.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: c8e6ad08

 ("ipv6: honor IPV6_PKTINFO with v4 mapped addresses on sendmsg")
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>