Add libevent to filter-deps, fix OpenMPI hook
Created by: bedroge
OpenMPI only allows to have an external PMIx AND libevent. We already had the first in filter-deps
, I'm now adding libevent there as well, so it doesn't get installed by EasyBuild. Also fixed the hook, as the correct line for OpenMPI with the proper path to PMIx was commented out (which makes OpenMPI build its own PMIx version).