separate targt ffdecsa-clean

Hi,

in the Gentoo ebuild (https://github.com/lucianm/gen2ovl-googoo2/blob/master/media-plugins/vdr-dvbapi/vdr-dvbapi-9999.ebuild), it comes handy to be able to clean the ffdecsa separately (you may look after 'emake clean-ffdecsa' in that link), when iterating through all PARALLEL_MODES in order to find out the best setting. Would be nice if you adopt this patch, thanks!

Ciao, Lucian