Add `eigen` as a dependency to `velodyne_pointcloud`

> This removes a dependency of `velodyne_pointcloud` that is still necessary in ROS 2 (see https://github.com/lopsided98/nix-ros-overlay/issues/511#issuecomment-2441025459 for an example of an issue this causes) 

 _Originally posted by @quantum9Innovation in [cd47706](https://github.com/ros-drivers/velodyne/commit/cd47706f8a57c46477b536e9749f372c7e75f40b#r148505456)_