ospf6d: fix warnings from recent prefix bit commit

* lib/prefix.h
  * prefix6_bit(): add IPv6 wrapper for prefix_bit()
* ospf6d/ospf6_lsdb.c
  * ospf6_lsdb_type_router_head(): employ prefix6_bit()
  * ospf6_lsdb_type_head(): idem
diff --git a/lib/prefix.h b/lib/prefix.h
index d370720..a7598b7 100644
--- a/lib/prefix.h
+++ b/lib/prefix.h
@@ -137,6 +137,12 @@
   return (prefix[offset] >> shift) & 1;
 }
 
+static inline unsigned int
+prefix6_bit (const struct in6_addr *prefix, const u_char prefixlen)
+{
+  return prefix_bit((const u_char *) &prefix->s6_addr, prefixlen);
+}
+
 /* Prototypes. */
 extern int afi2family (int);
 extern int family2afi (int);