Inject libprotobuf and libabseil dependency if a package depends on Gazebo Classic

[traversaro]

Workaround for:

• https://github.com/RoboStack/ros-humble/issues/175
• https://github.com/conda-forge/gazebo-feedstock/issues/107

Similar to https://github.com/RoboStack/ros-noetic/pull/464 .

It will be only useful after a rebuild, but let's do it so it is there when we do a rebuild.