ospf6d: add support for route tags

[ported by Paul Jakma, paul@jakma.org]
diff --git a/ospf6d/ospf6_route.h b/ospf6d/ospf6_route.h
index 2fb2c1d..027a648 100644
--- a/ospf6d/ospf6_route.h
+++ b/ospf6d/ospf6_route.h
@@ -97,6 +97,7 @@
   u_int8_t metric_type;
   u_int32_t cost;
   u_int32_t cost_e2;
+  u_int32_t tag;
 };
 
 #define OSPF6_PATH_TYPE_NONE         0