message: fix Windows dll import/export issues
diff: #include "ospcommon/common.h"
  #ifdef _WIN32
  -# ifdef ospray_module_mpi_EXPORTS
  +# ifdef ospray_mpi_common_EXPORTS
  # define OSPRAY_MPI_INTERFACE __declspec(dllexport)
  # else
  # define OSPRAY_MPI_INTERFACE __declspec(dllimport)
  