CAFFE2_INCLUDE_DIRS points to invalid path (#14306)

CAFFE2_INCLUDE_DIRS points to invalid path (#14306)

Summary:
I know that including CAFFE2_INCLUDE_DIRS in include headers are not necessary for newer cmakes. But I had this in one of my old projects and **cmake gave me error that "/usr/lib/include" is invalid path**.

It seems like "${_INSTALL_PREFIX}/lib/include" should be changed to "${_INSTALL_PREFIX}/include" as all caffe2 headers are in /include rather than /lib/include/

Please correct me if I am wrong?
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14306

Differential Revision: D13356246

Pulled By: ezyang

fbshipit-source-id: e2d5d3c42352e59b245714ad90fd7a9ef48170d7