mtd-utils: clean up zlib.h usage a bit

mtd-utils: clean up zlib.h usage a bit

Hide zlib's crc32 in compr_zlib.c and mkfs.ubifs/compr.c.

jffs2reader.c and mkfs.jffs2.c don't actually use zlib, so punt the
include from the file.

mkfs.jffs2.c is implicitly using crc32 from zlib.h instead of the
local mtd_crc32, so fix the local usage.  otherwise we get warnings
about undefined crc32 because the file was redirecting the prototype.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>