Correct an error message after waitpid

Usually we print the PID we wait for, let's do the same here.
Probably just a typo.

Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/namespaces.c

@@ -896,7 +896,7 @@ static int check_user_ns(int pid)
 	}
 
 	if (waitpid(chld, &status, 0) != chld) {
-		pr_perror("Unable to wait the %d process", pid);
+		pr_perror("Unable to wait for PID %d", chld);
 		return -1;
 	}