vdr 2.1.2 buildfix

a trivial one. from vdr changelog: 

+ The functions OpenVideoFile(), RenameVideoFile(), RemoveVideoFile(), VideoFileSpaceAvailable(),
VideoDiskSpace(), RemoveEmptyVideoDirectories(), IsOnVideoDirectoryFileSystem() and
PrefixVideoFileName() are now static members of cVideoDirectory and need to be called
with the proper prefix.
+ The name of the video directory is now available through cVideoDirectory::Name().