[caffe2] Switch to using `public_include_directories
Summary:
caffe2 uses `-I` all over the place, but really we should use the Buck built-in version of this
Alternatively, the `exported_header` clean up means we need to standardize to a single path
Test Plan:
```
buck build caffe2:torch-cpp-cpu
buck build caffe2/...
```
Reviewed By: malfet
Differential Revision: D19150098
fbshipit-source-id: e99aaf69d6c474afaedbd5f693a7736d3d67aafc
