[POWERPC] Fix build failure on Cell when CONFIG_SPU_FS=y

Commit aed3a8c9 introduced a
definition of notify_spus_active in .../cell/spu_syscalls.c, and
another definition under #ifndef MODULE in .../cell/spufs/sched.c.
The latter is not necessary and causes the build to fail when
CONFIG_SPU_FS=y, so this removes it.  It also removes the export
of do_notify_spus_active, which is unnecessary.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jeremy Kerr <jk@ozlabs.org>