v4ov6: enable unsupported nodes to announce v4ov6 routes

In the following topology

A --[v4]-- B --[v6]-- C

if B does not support v4ov6 (eg. because of an old kernel), it can
nevertheless announce a v4-over-v6 route to C, since the route in its
own routing table is pure v4.

message.c

@@ -1222,8 +1222,6 @@ really_buffer_update(struct buffered *buf, struct interface *ifp,
 
     if(v4) {
         if(!ifp->ipv4) {
-            if(!has_v4ov6)
-                return;
             ae = AE_V4OV6;
         } else {
             ae = AE_IPV4;