Use &self instead of &mut self for poll_accept

[kvinwang]

I encountered trouble when implementing the Listener from rocket for VsockListener, because it uses &self.

impl rocket::listener::Listener for VsockListener {
    type Accept = (vsock::VsockStream, vsock::VsockAddr);

    type Connection = VsockConnection;

    async fn accept(&self) -> io::Result<Self::Accept> {
        let (stream, addr) = self.listener.accept().await?;
        Ok((stream, addr))
    }
...
}

The TcpListener from tokio is also using &self.

So, I think it should also use &self here.

[jalil-salame]

The BVT check is failing due to clippy unused_mut lints, and the commit message is missing an sign off and a body (which is why the commit message check is failing).

[jalil-salame]

@kvinwang the commit message is still missing (as is the sign off), please check the CI error messages: https://github.com/rust-vsock/tokio-vsock/actions/runs/12293688962/job/34306956076?pr=54

[kvinwang]

@kvinwang the commit message is still missing (as is the sign off)

Fixed.