Dependencies missing in manifest

Dieses Issue hat noch niemand übernommen.

**Describe the bug**
Dependencies [declared as required in cmake](https://github.com/micro-ROS/micro-ROS-Agent/blob/galactic/micro_ros_agent/CMakeLists.txt#L32-L50) are missing [in the manifest file](https://github.com/micro-ROS/micro-ROS-Agent/blob/galactic/micro_ros_agent/package.xml#L18-L22).

**To reproduce**
I encountered this issue when trying to revive an old micro-ROS agent (foxy) installation on my ubuntu PC. Following [this tutorial](https://micro.ros.org/docs/tutorials/core/first_application_linux/), I
1. Pulled the micro_ros_setup repo
2. Executed create_agent_ws
3. Executed build_agent
---- Up to here it is expected behavior, the agent is built properly.

When I then execute `colcon build` in my ROS 2 workspace, the build of the micro_ros_agent package fails as it is missing microxrcedds_agent. When I place the microxrcedds_agent repo in my workspace, it will still fail on the first colcon run, since microxrcedds_agent is not specified as dependency in the micro-ROS agent package manifest, so colcon doesn't know to build it first.

**Expected behaviour**
All ROS 2 packages in my workspace successfully build

**System information (please complete the following information):**
Ubuntu 20.04, ROS 2 foxy, latest foxy branch

Vergleiche die in micro_ros_agent/CMakeLists.txt in den Zeilen 32-50 deklarierten Abhängigkeiten mit micro_ros_agent/package.xml in den Zeilen 18-22. Lies zunächst beide Dateien und führe dann colcon build in einem ROS 2-Workspace aus, der micro_ros_agent und microxrcedds_agent enthält. Erledigt ist die Aufgabe, wenn die Pakete erfolgreich in Abhängigkeitsreihenfolge gebaut werden.

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