message: fix a concat related build issue with old GCC version
diff: @@ -870,7 +870,7 @@ class MklConcatOp : public OpKernel {
  int concat_dim, bool* is_reorder_needed, int64* concat_dim_size) {
  *is_reorder_needed = false;
  *concat_dim_size = 0;
  - std::unordered_map<memory::format, int> occurrence_map;
  + std::unordered_map<int, int> occurrence_map;
  if (input_shapes.size() == 0)
  return memory::format::any;
  @@ -878,7 +878,7 @@ class MklConcatOp : public OpKernel {
  for (int k=0; k <input_shapes.size(); k++) {
  auto src_dims = TFShapeToMklDnnDims(input_shapes[k].GetTfShape());
  *concat_dim_size += src_dims[concat_dim];
  - memory::format fmt = static_cast<memory::format>(
  + int fmt = static_cast<int>(
  input_shapes[k].GetMklLayout().data.format);
  occurrence_map[fmt] += 1;
  }
  @@ -898,7 +898,7 @@ class MklConcatOp : public OpKernel {
  *is_reorder_needed = true;
  for (auto item : occurrence_map) {
  if (item.second > max_occurrence) {
  - commonest_format = item.first;
  + commonest_format = static_cast<memory::format>(item.first);
  max_occurrence = item.second;
  }
  }
  