[rqt_cm] Add CM as dependency (backport #1447)

[mergify]

If controller_manager itself is not built, then one gets an error at launching rqt_controller_manager

    from controller_manager.controller_manager_services import (
ModuleNotFoundError: No module named 'controller_manager'

imported here

ros2_control/rqt_controller_manager/rqt_controller_manager/controller_manager.py

Lines 20 to 26 in 20e01d8

	from controller_manager.controller_manager_services import (
	configure_controller,
	list_controllers,
	load_controller,
	switch_controllers,
	unload_controller,
	)

---

This is an automatic backport of pull request #1447 done by Mergify.