Include <string.h> to get memset and strcmp prototype.

Sort includes.

This is now WARNS=2 clean, bump WARNS to keep it clean.

usr.bin/hexdump/Makefile

@@ -7,5 +7,6 @@ MAN=	hexdump.1 od.1
 MLINKS=	hexdump.1 hd.1
 LINKS=	${BINDIR}/hexdump ${BINDIR}/od
 LINKS+=	${BINDIR}/hexdump ${BINDIR}/hd
+WARNS?=	2
 
 .include <bsd.prog.mk>

usr.bin/hexdump/conv.c

@@ -40,10 +40,11 @@ __FBSDID("$FreeBSD$");
 #include <sys/types.h>
 
 #include <assert.h>
-#include <stdio.h>
 #include <ctype.h>
 #include <limits.h>
+#include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <wchar.h>
 #include <wctype.h>
 #include "hexdump.h"