message: fix include after flatten
diff: #include <stan/math/prim/core/init_threadpool_tbb.hpp>
  #ifdef STAN_MPI
  -#include <stan/math/prim/arr/functor/mpi_cluster.hpp>
  -#include <stan/math/prim/arr/functor/mpi_command.hpp>
  -#include <stan/math/prim/arr/functor/mpi_distributed_apply.hpp>
  +#include <stan/math/prim/functor/mpi_cluster.hpp>
  +#include <stan/math/prim/functor/mpi_command.hpp>
  +#include <stan/math/prim/functor/mpi_distributed_apply.hpp>
  #endif
  // forward declaration for function defined in another translation unit
  