sh: fix up CONFIG_KEXEC=n build.

The reserve_crashkernel() definition is in asm/kexec.h which is only
dragged in via linux/kexec.h if CONFIG_KEXEC is set. Just switch over to
asm/kexec.h unconditionally to fix up the build.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>