dnsdist: Generate files when building the release tarball

dnsdist: Generate files when building the release tarball

Some build systems are very unhappy with `dnsdist-rust-lib/rust/src/lib.rs`
not existing before actually starting the build process (mostly because
of `cargo-vendor`), so let's make their life easier.

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>