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>