regression: broken ppc64le build with CONFIG_KERNEL_XZ=y

[Michal Kubecek]

Building 4.19-rc1 kernel with CONFIG_KERNEL_XZ=y fails with this error:

  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>
          ^~~~~~~~~~~~~~~~~~~

This is caused by commit faa16bc404d7 ("lib: Use existing define with
polynomial") which added include of <linux/crc32poly.h>; when compiling
arch/powerpc/boot/decompress.c, the include/ directory is not searched
so that the header file is not found.

The solution is probably adding -I directive to some place in
arch/powerpc/boot/Makefile but I don't dare to guess where.

Michal Kubecek