Use cmath instead of math.h (#1129)
Use cmath instead of math.h (#1129)

* Use cmath instead of math.h

Should fix issues with std I just saw in the pytorch builds. For example:

https://ci.pytorch.org/jenkins/job/pytorch-builds/job/pytorch-linux-trusty-py3.6-gcc5.4-build/10549/console
```
/var/lib/jenkins/workspace/third_party/onnx/onnx/common/tensor.h:342:40: error: 'sqrt' is not a member of 'std'
```

* retest

onnx/onnx,onnx/onnx,onnx/onnx,onnx/onnx