Update output_structure.cpp

variable definition bug for MPI Processing

[NOTE]
It is also needed to fix line 2221 as:
#ifdef WINDOWS
  int rank, iProcessor, nProcessor;
  MPI_Comm_rank(MPI_COMM_WORLD,&rank);
  MPI_Comm_size(MPI_COMM_WORLD,&nProcessor);
#else  
  int rank = MPI::COMM_WORLD.Get_rank();
  int nProcessor = MPI::COMM_WORLD.Get_size();
  int iProcessor;
#endif