powerpc/boot: Fix missing crc32poly.h when building with KERNEL_XZ

powerpc/boot: Fix missing crc32poly.h when building with KERNEL_XZ

Origin: https://patchwork.ozlabs.org/patch/963258/

After commit faa16bc404d7 ("lib: Use existing define with
polynomial") the lib/xz/xz_crc32.c includes a header from include/linux
directory thus any other user of this code should define proper include
path.

This fixes the build error on powerpc with CONFIG_KERNEL_XZ:

    In file included from ../arch/powerpc/boot/../../../lib/decompress_unxz.c:233:0,
                     from ../arch/powerpc/boot/decompress.c:42:
    ../arch/powerpc/boot/../../../lib/xz/xz_crc32.c:18:29: fatal error: linux/crc32poly.h: No such file or directory

Reported-by: Michal Kubecek <mkubecek@suse.cz>
Fixes: faa16bc404d7 ("lib: Use existing define with polynomial")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Meelis Roos <mroos@linux.ee>
Tested-by: Michal Kubecek <mkubecek@suse.cz>
Gbp-Pq: Topic bugfix/powerpc
Gbp-Pq: Name powerpc-boot-fix-missing-crc32poly.h-when-building-with-kernel_xz.patch