This is probably the most frequently asked question over on the wu-ftpd list :-)
Easy way: compile ls from sources, but use gcc -static at the end so it doesn't need shared libs. Works on all systems. Hard way: update a *lot* more of lib, you need ld.so and/or ld-linux.so, and you need to run ldconfig on that copy of lib (see the ldconfig man page for details.) The specific details vary a lot from system to system. (There's something to be said for having the debian ftpd package either handle this or include a script to do so... even if it means "dpkg --root ~ftp -i base*.deb" :-)