making sure bag_plugin is installed

Looks like the bag plugin for rqt_robot_monitor in 0.4.0 was created but not installed so it wasn't working properly.
Testing was done on the source so we missed it.
Since there might be more coming in the future please list them one per line and in alphabetical order:

    install(FILES
      bag_plugin.xml
      plugin.xml
      DESTINATION ${CATKIN_PACKAGE_SHARE_DESTINATION}
    )