t/io_uring: avoid unused `nr_batch` warning

t/io_uring: avoid unused `nr_batch` warning

If we have libaio support, but not an appropriate CPU clock, then the
build throws a warning on nr_batch being assigned but never used.

Mirror what was done on the io_uring init path and only defined and
set `nr_batch` if we have CPU clock support.

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