Ignore buggy clippy::large_enum_variant lint

```
error: large size difference between variants
   --> src/lib.rs:354:1
    |
354 | / pub enum TlsStream<T> {
355 | |     Client(client::TlsStream<T>),
    | |     ---------------------------- the second-largest variant contains at least 0 bytes
356 | |     Server(server::TlsStream<T>),
    | |     ---------------------------- the largest variant contains at least 1504 bytes
357 | | }
    | |_^ the entire enum is at least 0 bytes
    |
    = note: `-D clippy::large-enum-variant` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#large_enum_variant
```

Author: taiki-e

src/lib.rs

@@ -350,6 +350,7 @@ impl<IO: AsyncRead + AsyncWrite + Unpin> Future for FallibleAccept<IO> {
 ///
 /// This abstracts over the inner `client::TlsStream` and `server::TlsStream`, so you can use
 /// a single type to keep both client- and server-initiated TLS-encrypted connections.
+#[allow(clippy::large_enum_variant)] // https://github.com/rust-lang/rust-clippy/issues/9798
 #[derive(Debug)]
 pub enum TlsStream<T> {
     Client(client::TlsStream<T>),