Bug fix. Substitute return statement at the end of main with a call to

MPI_Finalize. Now this is a valid MPI program.

[[Imported from SVN: r1585]]

dune/istl/test/matrixredisttest.cc

@@ -144,5 +144,5 @@ int main(int argc, char** argv)
   }
 
   testRepart<1>(N,coarsenTarget);
-  return 0;
+  MPI_Finalize();
 }