Fix function declaration discrepancy in rt_thread

[carlossvg]

Closes #83

[LanderU]

@carlossvg, I think we need to backport this to foxy as well:

• https://github.com/ros2-realtime-demo/pendulum/blob/foxy/pendulum_utils/include/pendulum_utils/rt_thread.hpp#L37
• https://github.com/ros2-realtime-demo/pendulum/blob/foxy/pendulum_utils/include/pendulum_utils/rt_thread.hpp#L39