Add `strip` to release binaries

[Sascha Lüdecke]

What's your idea?

Not a big deal, but it reduces resulting file size of a release build on my Linux machine from 37M down to 25M by adding this to Cargo.toml:

[profile.release]
strip = true # strip binaries to reduce their size

[Matthew Kennerly]

Makes sense 👍 Rust recently enabling stripping in release mode by default, but it's probably still good to specify it since the Flatpak builds can use a slightly older version.