use different cmake variable for eigen include dir

Eigen3.2's upstream FindEigen3.cmake specifies only
EIGEN3_INCLUDE_DIR, not EIGEN3_INCLUDE_DIRS:
https://bitbucket.org/eigen/eigen/src/cafa6d909f409f4ec33070ebf991a77e7b747dc1/cmake/FindEigen3.cmake?at=default&fileviewer=file-view-default#FindEigen3.cmake-10

Eigen3.3-beta provides an Eigen3Config that defines
EIGEN3_INCLUDE_DIRS AND _DIR, but that's still beta, even though
Ubuntu introduced it in xenial already. So to stay backwards
compatible, _INCLUDE_DIR should be used.

kinect2_registration/CMakeLists.txt

@@ -41,7 +41,7 @@ endif()
 
 if(Eigen3_FOUND)
   message(STATUS "CPU based depth registration enabled")
-  include_directories(${EIGEN3_INCLUDE_DIRS})
+  include_directories(${EIGEN3_INCLUDE_DIR})
   set(DEPTH_REG_CPU ON)
   add_definitions(-DDEPTH_REG_CPU)
 else()