[POWERPC] cell: Fix spu_info.h header export

It uses #ifdef __KERNEL__, so needs to be processed with unifdef.

Signed-off-by: Arnd Bergann <arnd.bergmann@de.ibm.com>
Acked-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>