Add define for nlmsg_types with timestamp

Add #define for nlmsg_type = 15

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>
diff --git a/bridge/monitor.c b/bridge/monitor.c
index 76e7d47..f00e0a6 100644
--- a/bridge/monitor.c
+++ b/bridge/monitor.c
@@ -74,14 +74,12 @@
 			fprintf(fp, "[MDB]");
 		return print_mdb(who, n, arg);
 
-	case 15:
+	case NLMSG_TSTAMP:
 		return show_mark(fp, n);
 
 	default:
 		return 0;
 	}
-
-
 }
 
 int do_monitor(int argc, char **argv)