add libeigen3-dev for debian:squeeze

libeigen3-dev isn't officially supported for squeeze, but manually installing this package from the .deb proceeds without issue with a pretty stock squeeze. Including it in the base.yaml means that squeeze users can use eigen stack.

See my 'answers' post at http://answers.ros.org/question/130726/rosdep-fails-to-find-libeigen3-dev-on-debiansqueeze/