Fix missing variable declaration in hwloc 1.5.2

git-svn-id: https://svn.open-mpi.org/svn/ompi/trunk@28293 63e3feb5-37d5-0310-a306-e8a459e722fe
Add initial support for the MPI 3.0 tools interface (MPI_T).

Currently MPI_T support:
 - Full cvar interface.
 - Full categories interface.
 - No pvar support at this time.
Fix several bugs in variable registration.
MPIT: add a missing file