cerbero: Don't overwrite self.files_bins_devel
cerbero: Don't overwrite self.files_bins_devel

Create it if it exists, else append to it.

fluendo/cerbero,atsushieno/cerbero,GStreamer/cerbero,nirbheek/cerbero,fluendo/cerbero,atsushieno/cerbero,fluendo/cerbero,GStreamer/cerbero,atsushieno/cerbero,atsushieno/cerbero,fluendo/cerbero,GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,fluendo/cerbero,nirbheek/cerbero,nirbheek/cerbero,atsushieno/cerbero,GStreamer/cerbero