Add a version to the example/fixture dependencies on uniffi
This should allow to vendor the crates without hacks like
https://searchfox.org/mozilla-central/rev/fb55a4ee44a9a95d099aa806ca494eb988252ded/Cargo.toml#192-197
The downside is that it requires updating the version when the uniffi
version bumps in a semver-incompatible way.
