Fix clang warning from Thruster plugin

[chapulina]

🦟 Bug fix

• Follow up to Makes thruster stop when battery runs out. #1495

Summary

Just fixing a small warning that was introduced in #1495.

See https://build.osrfoundation.org/job/ignition_gazebo-ci-ign-gazebo6-homebrew-amd64/100/clang/

'PostUpdate' overrides a member function but is not marked 'override'

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

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[codecov]

Codecov Report

Merging #1540 (acc8da9) into ign-gazebo6 (c4dca78) will not change coverage.
The diff coverage is n/a.

❗ Current head acc8da9 differs from pull request most recent head bda2e36. Consider uploading reports for the commit bda2e36 to get more accurate results

@@             Coverage Diff              @@
##           ign-gazebo6    #1540   +/-   ##
============================================
  Coverage        64.03%   64.03%           
============================================
  Files              316      316           
  Lines            25462    25462           
============================================
  Hits             16304    16304           
  Misses            9158     9158           

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