northd.c: Fix nbcfg timestamp - use time_wall_msec instead of time_msec.

ovn-nbctl --print-wait-time is now broken due to this bug.
E.g.:
$ ovn-nbctl --wait=hv --print-wait-time sync
Time spent on processing nb_cfg 1:
	ovn-northd delay before processing:	-1643828314999ms
	ovn-northd completion:			-1643828314998ms
	ovn-controller(s) completion:		3ms

Fixes: b44f637 ("northd: Do not calculate database sequence numbers incrementally")
Signed-off-by: Han Zhou <hzhou@ovn.org>
Acked-by: Mark Michelson <mmichels@redhat.com>

northd/ovn-northd.c

@@ -816,7 +816,7 @@ main(int argc, char *argv[])
             }
 
             if (ovsdb_idl_has_lock(ovnsb_idl_loop.idl)) {
-                int64_t loop_start_time = time_msec();
+                int64_t loop_start_time = time_wall_msec();
                 inc_proc_northd_run(ovnnb_txn, ovnsb_txn, recompute);
                 recompute = false;
                 if (ovnsb_txn) {