linux: stub io_uring_setup.

[philmb3487_proton.me]

Summary

linux: stub io_uring_setup.

Cmake on musl libc fails if io_uring_setup doesn't exist. This is
a system call that was added around ~2019 that adds support for
io_uring, but io_uring itself is not always enabled on modern Linux
distros. Therefore, stub with (EPERM) to let userspace know what to
do.

Signed-off-by: Philippe Michaud-Boudreault <[e-mail]>

[philmb3487_proton.me]

Based on feedback, remove comment text to be more neutral.

[philmb3487_proton.me]

Reworked the commit message, and removed the function's comment as a follow up to feedback, in order to be more neutral.