Use assimp-dev dep for building

The assimp-dev dep has existed for a while now. We should use that for building and leave the runtime dep as-is.

See https://github.com/ros/rosdistro/commit/a506ef945febc0cd886bbc29ab976751d1cb1f3b

Blocks https://github.com/ros-visualization/rviz/issues/730 for Fedora