Disable building rtla, since bullseye's libtraceevent and libtracefs are too old

The build-dependencies for rtla aren't currently versioned, but the
upstream Makefile checks for libtraceevent >= 1.5 and libtracefs >=
1.3 which are not satisfiable in bullseye.  For now, disable building
rtla.  This can be reverted if newer versions of those libraries are
added to bullseye-backports.

See discussion at merge request !539.