arch-ppc.h: Fix typo in header guard

Signed-off-by: Akash Verma <akashv@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
ppc: disable CPU clock until we can detect whether we have it or not

The child segfault test should catch it, however it does not on
AIX at least.

Signed-off-by: Jens Axboe <axboe@kernel.dk>