Activate rustc args on docs.rs to fix the builds

Since the dox feature became obsolete, it is now needed to activate some rustc args when building the documentation on docs.rs. They are already present in the Cargo.toml file of the wrapper, but are missing from the sys crate. The builds fail without them, as can be seen here: https://docs.rs/crate/libadwaita-sys/0.5.2/builds