fix 'CallbackReturn' does not name a type

[ijnek]

Building on rolling, (and perhaps humble too), the following error shows up:

/home/ijnek/tmp_workspaces/dynamixel_control_ws/src/dynamixel_control/dynamixel_hardware/src/dynamixel_hardware.cpp:51:1: error: ‘CallbackReturn’ does not name a type
   51 | CallbackReturn DynamixelHardware::on_init(const hardware_interface::HardwareInfo & info)
      | ^~~~~~~~~~~~~~

This change fixes this error.

[youtalk]

Thank you for your PR. I really appreciate it but I cannot try the PR on ROS 2 Rolling environment right now.
That's why I've changed the merge branch from the main to the rolling.
After testing in my environment, I promise to merge to the main branch. Thank you again.

[ijnek]

Hi @youtalk, thank you for your existing work and the ongoing maintenance!