Don't use empty enum for opaque FFI type anymore

We have:

https://github.com/Boscop/web-view/blob/00bbb3adfd152f0509a9e577b78572c0a26f1a0f/webview-sys/lib.rs#L13

Even though the book was recommending to use empty enums for opaque FFI pointer types, it's not recommended anymore because it turned out it's UB:

https://www.reddit.com/r/rust/comments/a3bf2p/patterns_of_refactoring_c_to_rust_the_case_of/eb5xqje/

So it should be
```rust
#[repr(C)] pub struct CWebView { _private: [u8; 0] }
```