Duplicate platform_impl with build target wasm_unknown_emscripten

When running with:
```bash 
cargo build --target wasm_unknown_emscripten
```
Seems to duplicate platform_impl in https://github.com/RustAudio/cpal/blob/master/src/platform/mod.rs

#[cfg(target_os = "emscripten")]
mod platform_impl {

#[cfg(all(target_arch = "wasm32", feature = "wasm-bindgen"))]
mod platform_impl {
