[POWERPC] spufs: fix Makefile for "make clean"

added spu_{save,restore}_dump.h to target of 'make clean'

Signed-off-by: Masato Noguchi <Masato.Noguchi@jp.sony.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>