lib: fix "sockunion: add hash function" for BSD (9196caf)

lib: fix "sockunion: add hash function" for BSD (9196caf)

The sockunion_hash() function uses s6_addr32, which is not defined on
BSD systems.  (It only works on glibc because we set _GNU_SOURCE)

ripngd/ripng_nexthop.h already contains a workaround for this.  Bump
workaround to prefix.h so it's available everywhere.

Reported-by: NetDEF CI System <mwinter@netdef.org>
Fixes: 9196caf ("sockunion: add hash function")
Cc: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>