lib/decompress.c: consistency of compress formats for kernel image

Magic number of compress formats for kernel image is defined by two bytes.
 These numbers are written in hexadecimal number, nevertheless magic
number for only gunzip is written in octal number.  The formats should be
consistent for readability.  Therefore, magic numbers for gunzip are also
defined by hexadecimal number.

Signed-off-by: Haesung Kim <matia.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>