webrtc: add missing tokio feature for precise sync examples
requested to merge fengalin/gst-plugins-rs:webrtc-precise-sync-examples-tokio-signal-feature into main
Clippy caught the missing feature signal which is used by the WebRTC precise
synchronization examples. When running cargo check, build or clippy
without no-default-dependencies, this feature was already present due to
dependents crates.