[TCP] tcp_probe: use GCC printf attribute

The function in tcp_probe is printf like, use GCC to check the args.
Sighed-off-by: NStephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_probe.c

@@ -80,7 +80,8 @@ static void printl(const char *fmt, ...)
 
 	kfifo_put(tcpw.fifo, tbuf, len);
 	wake_up(&tcpw.wait);
-}
+} __attribute__ ((format (printf, 1, 2)));
+
 
 /*
  * Hook inserted to be called before each receive packet.