bgpd: attribute jhash call should use a standard interface to in6_addr data

* bgp_attr.c; (attrhash_key_make) s6_addr is only member of in6_addr
  guaranteed to be available - s6_addr32 isn't. Fix to be more portable, and
  thus allow compilation on BSD again.
diff --git a/bgpd/bgp_attr.c b/bgpd/bgp_attr.c
index e2b6054..7e30265 100644
--- a/bgpd/bgp_attr.c
+++ b/bgpd/bgp_attr.c
@@ -373,8 +373,8 @@
 
 #ifdef HAVE_IPV6
       MIX(attr->extra->mp_nexthop_len);
-      key = jhash2(attr->extra->mp_nexthop_global.s6_addr32, 4, key);
-      key = jhash2(attr->extra->mp_nexthop_local.s6_addr32, 4, key);
+      key = jhash2(attr->extra->mp_nexthop_global.s6_addr, 16, key);
+      key = jhash2(attr->extra->mp_nexthop_local.s6_addr, 16, key);
 #endif /* HAVE_IPV6 */
     }