sparc: Add missing "FORCE" target when using if_changed

Fix observed warning:

    /builds/linux/arch/sparc/boot/Makefile:35: FORCE prerequisite is missing

Fixes: e1f86d7b ("kbuild: warn if FORCE is missing for if_changed(_dep,_rule) and filechk")
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>