Fix cast.

src/xt_exchanger_irecv_isend.c

@@ -100,7 +100,7 @@ xt_exchanger_irecv_isend_a_exchange(const void *src_data, void *dst_data,
                           tmp_requests+i), comm);
 
   for (int i = 0; i < nsend; ++i)
-    xt_mpi_call(MPI_Isend((void *)src_data, 1, send_msgs[i].datatype,
+    xt_mpi_call(MPI_Isend(CAST_MPI_SEND_BUF(src_data), 1, send_msgs[i].datatype,
                           send_msgs[i].rank,
                           tag_offset + xt_mpi_tag_exchange_msg, comm,
                           tmp_requests+nrecv+i), comm);