Include benches directory in crate distribution

[badboy]

Otherwise the package is incomplete. And the Python source tarballs won't have this file and thus won't build.

[badboy]

Running make build-python-sdist in the Glean repository.
Then running this in a fresh Python virtual environment:

pip install path/to/glean/target/wheel/glean_sdk-66.2.0.tar.gz

takes a bit of time as it compiles the whole thing, but it then works.

<snip>
Successfully built glean-sdk
Installing collected packages: glean-sdk
Successfully installed glean-sdk-66.2.0