xtensa: boot: don't add include-dirs

Drop the -I<include-dir> options to prevent build warnings since there
is not boot/include directory:

cc1: warning: arch/xtensa/boot/include: No such file or directory [-Wmissing-include-dirs]

Fixes: 437374e9 ("restore arch/{ppc/xtensa}/boot cflags")
Fixes: 4bedea94 ("xtensa: Architecture support for Tensilica Xtensa Part 2")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Message-Id: <20230920052139.10570-15-rdunlap@infradead.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/boot/Makefile

@@ -9,8 +9,7 @@
 
 
 # KBUILD_CFLAGS used when building rest of boot (takes effect recursively)
-KBUILD_CFLAGS	+= -fno-builtin -Iarch/$(ARCH)/boot/include
-HOSTFLAGS	+= -Iarch/$(ARCH)/boot/include
+KBUILD_CFLAGS	+= -fno-builtin
 
 subdir-y	:= lib
 targets		+= vmlinux.bin vmlinux.bin.gz