lib/xz, lib/decompress_unxz.c: Fix spelling in comments

uncompressible -> incompressible
non-splitted -> non-split

Link: https://lore.kernel.org/r/20211010213145.17462-6-xiang@kernel.org


Signed-off-by: Lasse Collin <lasse.collin@tukaani.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>