powerpc: fix implicit use of mutex.h by include/asm/spu.h

We've been getting the header implicitly via module.h in the past
but when we clean that up, we'll get this failure:

  CC      arch/powerpc/platforms/cell/beat_spu_priv1.o
In file included from arch/powerpc/platforms/cell/beat_spu_priv1.c:22:
arch/powerpc/include/asm/spu.h:190: error: field 'list_mutex' has incomplete type
make[2]: *** [arch/powerpc/platforms/cell/beat_spu_priv1.o] Error 1

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