fix volumiologrotate if subfolders exist inside LOG_FOLDER.

[thegass]

after installation of squeezeboxserver i got error in journalctl because your logrotate doesn't recognize subfolders like /var/log/squeezeboxserver in LOG_FOLDER (/var/log/). no idea if find is the best solution to only check files but it solves the error.

[volumio]

Thanks!