hex_dump: add missing "const" qualifiers

Add missing "const" qualifiers to the print_hex_dump_bytes() library routines.

(akpm: rumoured to fix some compile warning somewhere)

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Cc: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>