This fixes a regression introduced by faa16bc4 ("lib: Use
existing define with polynomial").

The cleanup added a dependency on include/linux, which broke the PowerPC
boot wrapper/decompresser when KERNEL_XZ is enabled:

  BOOTCC  arch/powerpc/boot/decompress.o
 In file included from arch/powerpc/boot/../../../lib/decompress_unxz.c:233,
                 from arch/powerpc/boot/decompress.c:42:
 arch/powerpc/boot/../../../lib/xz/xz_crc32.c:18:10: fatal error:
 linux/crc32poly.h: No such file or directory
  #include <linux/crc32poly.h>
           ^~~~~~~~~~~~~~~~~~~

The powerpc decompresser is a hairy corner of the kernel. Even while building
a 64-bit kernel it needs to build a 32-bit binary and therefore avoid including
files from include/linux.

This allows users of the xz library to avoid including headers from
'include/linux/' while still achieving the cleanup of the magic number.

Fixes: faa16bc4

 ("lib: Use existing define with polynomial")
Reported-by: Meelis Roos <mroos@linux.ee>
Reported-by: kbuild test robot <lkp@intel.com>
Suggested-by: Christophe LEROY <christophe.leroy@c-s.fr>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Tested-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>