isisd: type mix-up in 28a8cfc "don't require IPv4"

isisd: type mix-up in 28a8cfc "don't require IPv4"

Whoops, these are in6_addrs, not prefix_ipv6... funnily enough, it does the
right thing either way, if it compiles, which it only does on Linux because
IN6_IS_ADDR_LINKLOCAL contains a cast to the right type.  On BSD there is no
such cast, hence it explodes on trying to compile, trying to access struct
members of in6_addrs while operating on prefix_ipv6...

Fixes: 28a8cfc ("isisd: don't require IPv4 for adjacency")
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>