gst-plugins-ugly: add legal info

Even though COPYING is LGPLv2.1+ many code files state v2+

Signed-off-by: Gustavo Zacarias <gustavo.zacarias@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>