Fix docs.rs build

Why is this important?

Whenever a crate is published on crates.io, its documentation is automatically built and linked. A documentation field is present in the Cargo.toml file so the default link to docs.rs is replaced by that link. I know that all gtk crates do that too, so maybe that was the idea. The downside of this is that the documentation is only available in the current version. If I use an older version, I can't have a look at its documentation (or it is not obvious how to do that). Luckily it is always possible to just go to docs.rs/CrateName and view the documentation there. On docs.rs it is possible to look at all previous versions. This is why there is value in having working builds on docs.rs. Having broken builds on docs.rs also prevents crates using this library from providing their documentation on docs.rs.

What is the issue?

Unfortunately the build of the current version is broken. I had a similar problem with one of my crates and I believe the reason is that the dependencies are not available. This should be possible to be fixed by activating the dox feature in the dependencies and the sys crate.