io_uring: uapi: Add `extern "C"` in io_uring.h for liburing

Make it easy for liburing to integrate uapi header with the kernel.
Previously, when this header changes, the liburing side can't directly
copy this header file due to some small differences. Sync them.

Link: https://lore.kernel.org/io-uring/f1feef16-6ea2-0653-238f-4aaee35060b6@kernel.dk


Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Dylan Yudaken <dylany@fb.com>
Cc: Facebook Kernel Team <kernel-team@fb.com>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>