Freetype backend doesn't work on macOS

I've enabled the freetype backend in font-kit like this:

```toml
font-kit = { version = "0.14.3", default-features = false, features = ["loader-freetype"] }
```

But it doesn't compile because the freetype-sys crate is missing at `src/loaders/freetype.rs:17:5`.
Enabling the `loader-freetype` feature should add the `freetype-sys` dependency regardless of platform.