Follow the pattern used with other *_show_fdinfo functions and only
define and use io_uring_show_fdinfo and its helper functions if
CONFIG_PROC_FS is set.

Fixes: 87ce955b ("io_uring: add ->show_fdinfo() for the io_uring file descriptor")
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Jens Axboe <axboe@kernel.dk>