calc_vmlinuz_load_addr.c requires SZ_64K to be defined for alignment
purposes.  It included "../../../../include/linux/sizes.h" to define
that size, however "sizes.h" tries to include <linux/const.h> which
assumes linux system headers.  These may not exist eg. the following
error was encountered when building Linux for OpenWrt under macOS:

In file included from arch/mips/boot/compressed/calc_vmlinuz_load_addr.c:16:
arch/mips/boot/compressed/../../../../include/linux/sizes.h:11:10: fatal error: 'linux/const.h' file not found
         ^~~~~~~~~~

Change makefile to force building on local linux headers instead of
system headers.  Also change eye-watering relative reference in include
file spec.

Thanks to Jo-Philip Wich & Petr Štetiar for assistance in tracking this
down & fixing.
Suggested-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org