get rid of warning about unused variable

[[Imported from SVN: r5930]]

dune/common/mpihelper.hh

@@ -209,7 +209,8 @@ namespace Dune
     {
       rank_ = -1;
       size_ = -1;
-      static int is_initialized = MPI_Init(&argc, &argv);
+      //static int is_initialized = MPI_Init(&argc, &argv);
+      MPI_Init(&argc, &argv);
       MPI_Comm_rank(MPI_COMM_WORLD,&rank_);
       MPI_Comm_size(MPI_COMM_WORLD,&size_);