Fix gmock CMP0115 (backport #1830)

[mergify]

Fixes

--- stderr: controller_manager                                           
CMake Warning (dev) at /opt/ros/rolling/share/ament_cmake_gmock/cmake/ament_add_gmock_executable.cmake:50 (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:

    /workspaces/ros2_rolling_ws/src/ros2_control/controller_manager/test/test_hardware_spawner.cpp
Call Stack (most recent call first):
  /opt/ros/rolling/share/ament_cmake_gmock/cmake/ament_add_gmock_executable.cmake:37 (_ament_add_gmock_executable)
  /opt/ros/rolling/share/ament_cmake_gmock/cmake/ament_add_gmock.cmake:65 (ament_add_gmock_executable)
  CMakeLists.txt:197 (ament_add_gmock)
This warning is for project developers.  Use -Wno-dev to suppress it.

---

This is an automatic backport of pull request #1830 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 434f7f5 has failed:

On branch mergify/bp/humble/pr-1830
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 434f7f5.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   controller_manager/CMakeLists.txt

no changes added to commit (use "git add" and/or "git commit -a")

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[christophfroehlich]

needs #1827 first

[christophfroehlich]

already included in #1827