fix warning on windows and don't also break unix

src/lib.rs

@@ -14,6 +14,9 @@ pub use mio_serial::{
 };
 
 use tokio::io::{AsyncRead, AsyncWrite, ReadBuf};
+
+#[cfg(unix)]
+use std::convert::TryFrom;
 use std::io::{Read, Result as IoResult, Write};
 use std::pin::Pin;
 use std::task::{Context, Poll};