Expose io_uring_enter flags using bitflags! macro.

[sugarraysam]

Addressing another issue uncovered in #363.

Flags are required to issue custom io_uring_enter syscalls:

    pub fn wait_cqes_timeout(&self, min_complete: u32, timeout: Duration) -> Result<usize> {
        let opts = flags::IORING_ENTER_EXT_ARG | flags::IORING_ENTER_GETEVENTS; <-- sys was private

        let ts = types::Timespec::from(timeout);
        let args = types::SubmitArgs::new().timespec(&ts);

        unsafe {
            self.ring
                .submitter()
                .enter(0, to_complete, opts, Some(&args.args))
        }
    }

[quininer]

I don't think this is a good export, please refer to the exist flag wrapping.

[sugarraysam]

I don't think this is a good export, please refer to the exist flag wrapping.

Ahh my bad I missed this bitflags! macro setup. Fixed !

[quininer]

Thank you!