Adding ros2_control dependency to demos (#74)

[CSharpRon]

One-line addition to the package.xml definition for the demos so that rosdep knows to install the ros2_control package.
Fixes #74

To Test

Since the Docker/Dockerfile performs a clone of the repository@master, you can replace this line locally with my repository and branch and then build and test with Docker.

sed -i 's@git clone https://github.com/ros-simulation/gazebo_ros2_control@git clone --single-branch --branch demo-dependency-fix https://github.com/CSharpRon/gazebo_ros2_control@' Docker/Dockerfile 
cd Docker
docker build -t gazebo_ros2_control_demos:test .
docker run gazebo_ros2_control_demos:test

[ahcorde]

Thank you @CSharpRon