try fix build ios

Signed-off-by: fufesou <shuanglongchen@yeah.net>

src/client/io_loop.rs

@@ -18,6 +18,7 @@ use hbb_common::protobuf::Message as _;
 use hbb_common::rendezvous_proto::ConnType;
 #[cfg(not(any(target_os = "android", target_os = "ios")))]
 use hbb_common::sleep;
+#[cfg(not(target_os = "ios"))]
 use hbb_common::tokio::sync::mpsc::error::TryRecvError;
 #[cfg(windows)]
 use hbb_common::tokio::sync::Mutex as TokioMutex;