lftp: use a download location that keeps old versions around

This changes the mirror for LFTP from ftp://ftp.cs.tu-berlin.de/pub/net/ftp/lftp/ to ftp://ftp.tuwien.ac.at/infosys/browsers/ftp/lftp/. The old mirror only had the latest version of lftp and would break every time a new version was released.