powerpc: fix implicit notifier use in converting to export.h

We can convert this file to using export.h since it only wants
to export symbols, but when we do we'll see also that it was
implicitly getting notifier.h from module.h via this failure:

  CC      arch/powerpc/platforms/cell/spu_notify.o
arch/powerpc/platforms/cell/spu_notify.c:28: warning: type defaults to 'int' in declaration of 'BLOCKING_NOTIFIER_HEAD'
arch/powerpc/platforms/cell/spu_notify.c:28: warning: parameter names (without types) in function declaration
arch/powerpc/platforms/cell/spu_notify.c: In function 'spu_switch_notify':
arch/powerpc/platforms/cell/spu_notify.c:32: error: implicit declaration of function 'blocking_notifier_call_chain'
arch/powerpc/platforms/cell/spu_notify.c:32: error: 'spu_switch_notifier' undeclared (first use in this function)

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>