Fixed target of ament_export_libraries (backport #295)

[mergify]

Since a non-existent library was specified for export, a warning was generated as shown below when building packages that depend on gz_ros2_control.

CMake Warning at /home/ubuntu/ws_gz_ros2_control/install/gz_ros2_control/share/gz_ros2_control/cmake/ament_cmake_export_libraries-extras.cmake:116 (message):
  Package 'gz_ros2_control' exports library 'gz_ros2_control' which couldn't
  be found
Call Stack (most recent call first):
  /home/ubuntu/ws_gz_ros2_control/install/gz_ros2_control/share/gz_ros2_control/cmake/gz_ros2_controlConfig.cmake:41 (include)
  CMakeLists.txt:49 (find_package)

This pull request changes it to export the correct existing target.

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This is an automatic backport of pull request #295 done by Mergify.