tweak to avoid "unused variable" warnings

bfps/cpp/particles/particles_distr_mpi.hpp

@@ -235,6 +235,7 @@ public:
         }
         const int nbProcToRecvUpper = int(neigDescriptors.size())-nbProcToRecvLower;
         const int nbProcToRecv = nbProcToRecvUpper + nbProcToRecvLower;
+        variable_used_only_in_assert(nbProcToRecv);
         assert(int(neigDescriptors.size()) == nbProcToRecv);
 
         for(int idxDescr = 0 ; idxDescr < int(neigDescriptors.size()) ; ++idxDescr){