Duplicate platform_impl with build target wasm_unknown_emscripten

[kokosha]

When running with:

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 {