powerpc/cell: Fix compile error in ras.c

This fixes this error on Cell when CONFIG_KEXEC = n:

arch/powerpc/platforms/cell/ras.c:299: error: implicit declaration of function 'crash_shutdown_register'

We have to include <asm/kexec.h> because it contains the dummy
definition of crash_shutdown_register that is used when
CONFIG_KEXEC=n, but <linux/kexec.h> doesn't include <asm/kexec.h> in
that case.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>