fix(notification): gate the OnceLock import to macOS

`std::sync::OnceLock` is only reachable from `NOTIFICATION_IDENTITY`, which is
macOS-only, but the import was gated on `tauri-runtime` alone — so every
non-macOS desktop build carried an unused import and failed the lint under
`-D warnings`. The import now carries the same `target_os` gate as its one
use site.