build fix on FreeBSD (#3132)

Signed-off-by: Steve Wills <steve@mouf.net>

collector/netdev_bsd.go

@@ -70,3 +70,7 @@ func getNetDevStats(filter *deviceFilter, logger *slog.Logger) (netDevStats, err
 
 	return netDev, nil
 }
+
+func getNetDevLabels() (map[string]map[string]string, error) {
+	return nil, nil
+}