vdr 2.1.2 buildfix (vnsiserver)

not sure if I should send this to @FernetMenta instead

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().