Fix gmock CMP0115

[christophfroehlich]

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.

[saikishor]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.25%. Comparing base (5dbd6d5) to head (5c32b18).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1830   +/-   ##
=======================================
  Coverage   88.25%   88.25%           
=======================================
  Files         121      121           
  Lines       12301    12301           
  Branches     1093     1093           
=======================================
  Hits        10856    10856           
  Misses       1051     1051           
  Partials      394      394           

Flag	Coverage Δ
unittests	88.25% <ø> (ø)

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