bindings: Consistently check for `dox` feature

bindings: Consistently check for `dox` feature

This fixes the build on docs.rs:

https://docs.rs/crate/ostree-ext/0.10.2/builds/699039
```
[INFO] running `Command { std: "docker" "start" "-a" "7d90466f040d987cbcc393bc89f5f0688f052f49e2339c354dd731fdaf0f9667", kill_on_drop: false }`
[INFO] [stderr]     Checking ostree v0.17.0
[INFO] [stderr] error[E0063]: missing field `process_passthrough_whiteouts` in initializer of `repo_checkout_at_options::RepoCheckoutAtOptions`
[INFO] [stderr]   --> /opt/rustwide/cargo-home/registry/src/github.com-1ecc6299db9ec823/ostree-0.17.0/rust-bindings/src/repo_checkout_at_options/mod.rs:61:9
[INFO] [stderr]    |
[INFO] [stderr] 61 |         RepoCheckoutAtOptions {
[INFO] [stderr]    |         ^^^^^^^^^^^^^^^^^^^^^ missing `process_passthrough_whiteouts`
```