Update Cargo.toml to declare libc dependency for target_os=emscripten

[burakemir]

When targetting wasm32-unknown-emscripten, there is a dependency on libc.
The Cargo.toml should reflect that.

[newpavlov]

IIUC Emscripten in included into the UNIX target OS family, so this is redundant. Note that we test Emscripten build in our CI.

[burakemir]

Thanks, you are right and there is an issue in our build configuration. Also figured out how to --print all-target-specs-json.

Closing this.