Avoid to simlulate the existance of std.

[0ndorio]

The current version of esp32-sys is required to simulate the existance of std::os::raw as the bindings are generated without the --cyptes-prefix flag. Without this flag any libc related types are automatically imported via ::std::os::raw.

This is also the reason why the esp32-sys crate couldn't be compiled with the edition = "2018" flag.

(see ctron/rust-esp-container#2 for the required change inside the template repository)