Add missing headers

[jennuine]

🦟 Bug fix

Fixes gz-gui7-focal

Summary

Added the missing headers which were causing examples/standalone/point_cloud/point_cloud.cc compilation to fail.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #486 (081ec02) into gz-gui7 (b5f354a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           gz-gui7     #486   +/-   ##
========================================
  Coverage    67.82%   67.82%           
========================================
  Files           44       44           
  Lines         4826     4826           
========================================
  Hits          3273     3273           
  Misses        1553     1553           

Impacted Files	Coverage Δ
src/plugins/camera_tracking/CameraTracking.cc	68.45% <ø> (ø)
src/plugins/grid_config/GridConfig.cc	22.35% <ø> (ø)
src/plugins/grid_config/GridConfig.hh	100.00% <ø> (ø)
src/plugins/image_display/ImageDisplay.cc	31.70% <ø> (ø)
...interactive_view_control/InteractiveViewControl.cc	17.88% <ø> (ø)
src/plugins/key_publisher/KeyPublisher.cc	56.52% <ø> (ø)
src/plugins/marker_manager/MarkerManager.cc	59.41% <ø> (ø)
src/plugins/minimal_scene/MinimalScene.cc	65.61% <ø> (ø)
src/plugins/publisher/Publisher.cc	90.78% <ø> (ø)
src/plugins/screenshot/Screenshot.cc	30.12% <ø> (ø)
... and 7 more

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