Fix #5874: Use of designated initializers requires C++20 fails the build

Designated initializers are a feature of C++20:
https://en.cppreference.com/w/cpp/language/aggregate_initialization.
However, they appeared in
https://github.com/google-ai-edge/mediapipe/blob/ffe429d5278b914c44fdb5df3ce38962b55580bb/mediapipe/framework/output_stream_handler.cc#L145-L149

The project uses C++17 and adapting it for C++20 would be non-trivial.
The easiest fix is to avoid using aggregated initialization.
It is fine while the order of members is preserved:
```cpp
struct OutputStreamMonitoringInfo {
  std::string stream_name;
  // The total number of packets added to the output stream.
  int num_packets_added;
  // The next timestamp bound of the output stream.
  Timestamp next_timestamp_bound;
};
```
https://github.com/google-ai-edge/mediapipe/blob/eecf8a5696d5e47edbd3b6e99a6064c99f9ab1a4/mediapipe/framework/output_stream_handler.h#L53-L59

Author: definability

mediapipe/framework/output_stream_handler.cc

@@ -143,9 +143,7 @@ OutputStreamHandler::GetMonitoringInfo() {
       continue;
     }
     monitoring_info_vector.emplace_back(OutputStreamMonitoringInfo(
-        {.stream_name = DebugStreamName(id),
-         .num_packets_added = stream->NumPacketsAdded(),
-         .next_timestamp_bound = stream->NextTimestampBound()}));
+        {DebugStreamName(id), stream->NumPacketsAdded(), stream->NextTimestampBound()}));
   }
   return monitoring_info_vector;
 }