Ignore pseudo-leaks when we use dlopen in smpi.

In a perfect world we should dlclose everything properly, but this leads to issues with some other atexit handles.
The xbt_postexit is always executed last, and closes the logs. If the SMPI code used XBT_LOG features, it is then needed until the last moment.
So we would need to store the handle at the XBT level instead of the SMPI one... And we don't really want that.
So we just hide these non-leaks from valgrind.