cerbero/source: Use ftp.gnu.org instead of ftpmirror
cerbero/source: Use ftp.gnu.org instead of ftpmirror

ftpmirror points to random GNU mirrors which are community maintained
and are likely to be down.

GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero,nirbheek/cerbero,nirbheek/cerbero,GStreamer/cerbero