Put EIGEN_ALIGN16 after 'union' keyword

The EIGEN_ALIGN16 macro preceded the union keyword in the code, gcc 4.8 however requires it to follow, as stated for example by this warning:
[...]/point_types.hpp:813:5: note: in expansion of macro ‘PCL_ADD_POINT4D’
[...]/point_types.hpp:124:9: note: attribute for ‘union tas::lidar::Point::<anonymous>’ must follow the ‘union’ keyword"