MIPS: fix build on non-linux hosts

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

arch/mips/boot/compressed/Makefile

@@ -78,6 +78,8 @@ OBJCOPYFLAGS_piggy.o := --add-section=.image=$(obj)/vmlinux.bin.z \
 $(obj)/piggy.o: $(obj)/dummy.o $(obj)/vmlinux.bin.z FORCE
 	$(call if_changed,objcopy)
 
+HOSTCFLAGS_calc_vmlinuz_load_addr.o += $(LINUXINCLUDE)
+
 # Calculate the load address of the compressed kernel image
 hostprogs-y := calc_vmlinuz_load_addr
 

arch/mips/boot/compressed/calc_vmlinuz_load_addr.c

@@ -9,7 +9,7 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include "../../../../include/linux/sizes.h"
+#include <linux/sizes.h>
 
 int main(int argc, char *argv[])
 {