Build fails on Windows

The same issue as #48 is back in v0.6.0.

```rs
error[E0433]: cannot find `fs` in `windows`
--> C:\Users\admin\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\async-tar-0.6.0\src\entry.rs:609:45
|
609 | async_std::os::windows::fs::symlink_file(src, dst).await
| ^^ could not find `fs` in `windows`
|
note: found an item that was configured out
--> C:\Users\admin\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\async-std-1.13.2\src\os\windows\mod.rs:9:13
|
9 | pub mod fs;
| ^^
|
::: C:\Users\admin\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\async-std-1.13.2\src\utils.rs:137:19
|
137 | #[cfg(feature = "unstable")]
| -------------------- the item is gated behind the `unstable` feature
```

In https://github.com/dignifiedquire/async-tar/commit/41e058705f60041a4cedde1de6b5d249ad78d579#diff-2e9d962a08321605940b5a657135052fbcef87b5e360662bb527c96d9a615542L23 `features = ["unstable"]` was removed from `async-std`.