Servo: add missing include

[v4hn]

The code uses std::atomic<bool> further below, but relies on other headers including it.
It's not always there though. Compiling failed for me on Lunar-Linux with ROS melodic.

[codecov]

Codecov Report

Merging #2519 (028ea1c) into master (ea50801) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2519      +/-   ##
==========================================
- Coverage   60.30%   60.26%   -0.03%     
==========================================
  Files         351      351              
  Lines       26471    26471              
==========================================
- Hits        15960    15951       -9     
- Misses      10511    10520       +9     

Impacted Files	Coverage Δ
moveit_core/robot_model/src/joint_model_group.cpp	63.08% <0.00%> (-2.46%)	⬇️
...e/src/parameterization/model_based_state_space.cpp	70.40% <0.00%> (-2.39%)	⬇️
...dl_kinematics_plugin/src/kdl_kinematics_plugin.cpp	76.55% <0.00%> (+0.45%)	⬆️
...on/pick_place/src/approach_and_translate_stage.cpp	72.96% <0.00%> (+0.82%)	⬆️

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[AndyZe]

Thanks!