My local build failed due to the patch to onnx

When I built onnx-mlir locally, I ran into errors like:
```
onnx-mlir/build/third_party/onnx/onnx/onnx-ml.pb.cc:9959:1: error: 'Arena' has not been declared
```

I think that they are caused by the temporary patch to onnx. @Sunny-Anand I saw the change in docker build file. Could you also add that change to docs/BuildOnLinuxOSX.md?