io_uring: unexport io_req_cqe_overflow()

There are no users of io_req_cqe_overflow() apart from io_uring.c, make
it static.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f4295eb2f9eb98d5db38c0578f57f0b86bfe0d8c.1712708261.git.asml.silence@gmail.com


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