Add .cc extension to suppress CMake warnings

[mjcarroll]

🦟 Bug fix

Summary

Newer versions of CMake explicitly require extensions on all files.

Without extensions, you get warnings:

CMake Warning (dev) at /home/mjcarroll/workspaces/gz_garden/install/share/cmake/gz-cmake3/cmake3/IgnUtils.cmake:1742 (add_executable):
  Policy CMP0115 is not set: Source file extensions must be explicit.  Run
  "cmake --help-policy CMP0115" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.

  File:

    /home/mjcarroll/workspaces/gz_garden/src/gz-gui/src/Application_TEST.cc
Call Stack (most recent call first):
  /home/mjcarroll/workspaces/gz_garden/install/share/cmake/gz-cmake3/cmake3/IgnUtils.cmake:1854 (gz_build_executables)
  src/CMakeLists.txt:46 (gz_build_tests)
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.

[ahcorde]

• you need to remove the .cc from line 43
• or add only .cc in line 48 and remove your changes

[codecov]

Codecov Report

Merging #428 (febafb0) into main (05f4bec) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #428      +/-   ##
==========================================
+ Coverage   64.03%   64.07%   +0.03%     
==========================================
  Files          45       45              
  Lines        5539     5539              
==========================================
+ Hits         3547     3549       +2     
+ Misses       1992     1990       -2     

Impacted Files	Coverage Δ
src/plugins/scene3d/Scene3D.cc	49.81% <0.00%> (+0.25%)	⬆️

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