MIPS: boot: Define __ASSEMBLY__ for its.S build

The MIPS %.its.S compiler command did not define __ASSEMBLY__, which meant
when compiler_types.h was added to kconfig.h, unexpected things appeared
(e.g. struct declarations) which should not have been present. As done in
the general %.S compiler command, __ASSEMBLY__ is now included here too.

The failure was:

    Error: arch/mips/boot/vmlinux.gz.its:201.1-2 syntax error
    FATAL ERROR: Unable to parse input tree
    /usr/bin/mkimage: Can't read arch/mips/boot/vmlinux.gz.itb.tmp: Invalid argument
    /usr/bin/mkimage Can't add hashes to FIT blob

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 28128c61 ("kconfig.h: Include compiler types to avoid missed struct attributes")
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>