io-uring: move io_wait_queue definition to header file

This moves the definition of the io_wait_queue structure to the header
file so it can be also used from other files.

Signed-off-by: Stefan Roesch <shr@devkernel.io>
Link: https://lore.kernel.org/r/20230608163839.2891748-4-shr@devkernel.io


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