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

After commit faa16bc ("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: faa16bc ("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>
Link: deepin-community#534
(cherry picked from commit 9ba3135758564489a9db0f7c70124c876283e401)
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>

Author: krzk

arch/powerpc/boot/Makefile

@@ -98,7 +98,7 @@ BOOTCFLAGS	+= -fno-stack-protector
 endif
 
 BOOTCFLAGS	+= -include $(srctree)/include/linux/compiler_attributes.h
-BOOTCFLAGS	+= -I$(objtree)/$(obj) -I$(srctree)/$(obj)
+BOOTCFLAGS	+= -I$(objtree)/$(obj) -I$(srctree)/$(obj) -I$(srctree)/include
 
 DTC_FLAGS	?= -p 1024