libpayload: hexdump.c: Change type of length argument to size_t

Representing a (non-negative) length with a signed integer is not
optimal, so change its type to `size_t`.

Change-Id: Ic0c2b7e081ba32d917409568ee53007d9ab7f8f3
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/4768
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

payloads/libpayload/libc/hexdump.c

@@ -29,7 +29,7 @@
 
 #include <libpayload.h>
 
-void hexdump(void *memory, int length)
+void hexdump(void *memory, size_t length)
 {
 	int i;
 	uint8_t *m;