libtailscale: set EventBus in wgengine.Config

Updates tailscale/tailscale#16369

Signed-off-by: Nick Khyl <nickk@tailscale.com>

libtailscale/backend.go

@@ -317,6 +317,7 @@ func (a *App) newBackend(dataDir string, appCtx AppContext, store *stateStore,
 		HealthTracker:  sys.HealthTracker(),
 		Metrics:        sys.UserMetricsRegistry(),
 		DriveForLocal:  driveimpl.NewFileSystemForLocal(logf),
+		EventBus:       sys.Bus.Get(),
 	})
 	if err != nil {
 		return nil, fmt.Errorf("runBackend: NewUserspaceEngine: %v", err)