Update FindFFmpeg to find/search for libswscale

[Christoph Feck]

Hello,

For the upcoming FFmpeg based thumbnailer, we need the libswscale library 
that is part of the FFmpeg project.

The FindFFmpeg.cmake in kdelibs does, however, not handle that library. 
Could somebody with knowledge of cmake voodoo update that file? It would 
be nice if an additional HAVE_SWSCALE is added to the results.

Possible candidates to look at are 
http://websvn.kde.org/trunk/kdesupport/strigi/libstreamanalyzer/cmake/FindFFMPEG.cmake?view=markup 
and http://www.openlibraries.org/browser/trunk/FindFFMPEG.cmake

The alternative would be to add a module-private FindFFMPEG.cmake, but I 
guess there are other projects that would benefit from the addition.

Thanks in advance,
Christoph Feck (kdepepo)