bgpd: fix printed value of last-update timestamp

* bgp_route.c: (route_vty_out_detail) calculate time value
    in a way, which works regardless of monotonic clock
    being used or not
diff --git a/bgpd/bgp_route.c b/bgpd/bgp_route.c
index 2391f74..1cfc451 100644
--- a/bgpd/bgp_route.c
+++ b/bgpd/bgp_route.c
@@ -6145,7 +6145,8 @@
 	bgp_damp_info_vty (vty, binfo);
 
       /* Line 7 display Uptime */
-      vty_out (vty, "      Last update: %s", ctime (&binfo->uptime));
+      time_t tbuf = time(NULL) - (bgp_clock() - binfo->uptime);
+      vty_out (vty, "      Last update: %s", ctime(&tbuf));
     }
   vty_out (vty, "%s", VTY_NEWLINE);
 }