Use EIGEN3_INCLUDE_DIR for Eigen includes
Use EIGEN3_INCLUDE_DIR for Eigen includes

Summary:
The plural version is not defined in the CentOS CMake module.
Verified EIGEN3_INCLUDE_DIR is defined in the Ubuntu CMake module.

This fixes the build on CentOS when using system Eigen3.
Closes https://github.com/caffe2/caffe2/pull/1505

Differential Revision: D6390712

Pulled By: pietern

fbshipit-source-id: b8abb14a62e0ff9fa9c920866504da0e75786c0d

sf-wind/caffe2,Yangqing/caffe2,pietern/caffe2,xzturn/caffe2,Yangqing/caffe2,sf-wind/caffe2,davinwang/caffe2,davinwang/caffe2,pietern/caffe2,davinwang/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,pietern/caffe2,pietern/caffe2,pietern/caffe2,Yangqing/caffe2,Yangqing/caffe2,sf-wind/caffe2,davinwang/caffe2,caffe2/caffe2,sf-wind/caffe2,xzturn/caffe2,xzturn/caffe2,davinwang/caffe2,sf-wind/caffe2