libpayload/libc/hexdump.c: Take const void *memory as argument

`*memory` is not changed in `hexdump()` and just read so make it
`const`.

Change-Id: I9504d25ab5c785f05c39c9a4f48c21f68659a829
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/5403
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)

payloads/libpayload/libc/hexdump.c

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