zebra: use prefix_ipv6 in rtadv_prefix

rtadv_prefix.prefix was casted to "struct prefix_ipv6" and had the same
size, make it exactly this type to make the code a bit cleaner.
diff --git a/zebra/rtadv.h b/zebra/rtadv.h
index d8d263d..5bf6b84 100644
--- a/zebra/rtadv.h
+++ b/zebra/rtadv.h
@@ -30,7 +30,7 @@
 struct rtadv_prefix
 {
   /* Prefix to be advertised. */
-  struct prefix prefix;
+  struct prefix_ipv6 prefix;
   
   /* The value to be placed in the Valid Lifetime in the Prefix */
   u_int32_t AdvValidLifetime;