Initialize antiwindup variable properly (backport #326)

[mergify]

/workspaces/ros2_baustelle_ws/src/ros-controls/gazebo_ros2_control/gazebo_ros2_control/src/gazebo_system.cpp: In member function ‘control_toolbox::Pid gazebo_ros2_control::GazeboSystem::extractPID(std::string, hardware_interface::ComponentInfo)’:
/workspaces/ros2_baustelle_ws/src/ros-controls/gazebo_ros2_control/gazebo_ros2_control/src/gazebo_system.cpp:231:93: warning: ‘antiwindup’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  231 |   return control_toolbox::Pid(kp, ki, kd, max_integral_error, min_integral_error, antiwindup);
      |                                                                                             ^
```<hr>This is an automatic backport of pull request #326 done by [Mergify](https://mergify.com).