Fix CMake warning in example

[azeey]

🦟 Bug fix

Summary

The add_library call was missing the file extension which caused a CMake warning on CI:

133: CMake Warning (dev) at CMakeLists.txt:13 (add_library):
133:   Policy CMP0115 is not set: Source file extensions must be explicit.  Run
133:   "cmake --help-policy CMP0115" for policy details.  Use the cmake_policy
133:   command to set the policy and suppress this warning.
133: 
133:   File:
133: 
133:     /home/jenkins/workspace/gz_sim-ci-pr_any-jammy-amd64/gz-sim/examples/plugin/hello_world/HelloWorld.cc
133: This warning is for project developers.  Use -Wno-dev to suppress it.

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 #2145 (2f46bbf) into gz-sim8 (d052491) will not change coverage.
Report is 1 commits behind head on gz-sim8.
The diff coverage is n/a.

❗ Current head 2f46bbf differs from pull request most recent head d1dbb42. Consider uploading reports for the commit d1dbb42 to get more accurate results

@@           Coverage Diff            @@
##           gz-sim8    #2145   +/-   ##
========================================
  Coverage    65.45%   65.45%           
========================================
  Files          323      323           
  Lines        30704    30704           
========================================
  Hits         20096    20096           
  Misses       10608    10608