fio: Eliminate compilation warning in ppc

Eliminate compilation warning : 'atb_clocktest' defined but
not used. Function is declared as static in header file. So compiler will
know if they used or not. Just eliminate the warning by #if 0 /unless we do
use it.

Signed-off-by: Sumanth K <sumantk2@linux.vnet.ibm.com>
Signed-off-by: Jens Axboe <axboe@fb.com>