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>
diff --git a/ripngd/ripng_nexthop.h b/ripngd/ripng_nexthop.h
index 7c04105..19bd32b 100644
--- a/ripngd/ripng_nexthop.h
+++ b/ripngd/ripng_nexthop.h
@@ -41,17 +41,8 @@
  * -1 if A < B
  **/
 static inline int
-addr6_cmp(struct in6_addr *A, struct in6_addr *B) {
-
-#ifndef s6_addr32
-#if defined(SUNOS_5)
-/* Some SunOS define s6_addr32 only to kernel */
-#define s6_addr32 _S6_un._S6_u32
-#else
-#define s6_addr32 __u6_addr.__u6_addr32
-#endif /* SUNOS_5 */
-#endif /*s6_addr32*/
-
+addr6_cmp(struct in6_addr *A, struct in6_addr *B)
+{
 #define a(i) A->s6_addr32[i]
 #define b(i) B->s6_addr32[i]