hexdump: use const notation

Trivial fix: mark the buffer to hexdump as const so callers could avoid
casting their const buffers when calling print_hex_dump().

The patch is really trivial and I suggest to consider it as a fix
(it fixes GCC warnings) and push it to current tree.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>