Dokumentation enhancement: Linux Dependencys

http://www.stepmania.com/wiki/linux-dependencies/

The configure script output is quite clear but the lines for testing video support are easy to overlook. So adding this line to the doku could help:

* for videoplayback in the songs you need also: libswscale-dev, libavformat-dev, libavcodec-dev, and libavutil-dev

ffmpeg is going to be bundled in the next release, so that shouldn't be an issue.