libsa/arp.c: Change printf format string

Change printf format string to avoid compilation failure when
ARP_DEBUG macro is defined.

Reviewed by:	imp
Obtained from:	Semihalf
Differential Revision: https://reviews.freebsd.org/D36735

stand/libsa/arp.c

@@ -178,7 +178,7 @@ arprecv(struct iodesc *d, void **pkt, void **payload, time_t tleft, void *extra)
 	if (n == -1 || n < sizeof(struct ether_arp)) {
 #ifdef ARP_DEBUG
 		if (debug)
-			printf("bad len=%d\n", n);
+			printf("bad len=%zd\n", n);
 #endif
 		free(ptr);
 		return (-1);