rename rosidl_generator_c namespace to rosidl_runtime_c (#540)
rename rosidl_generator_c namespace to rosidl_runtime_c (#540)

Signed-off-by: Dirk Thomas <19795c8185177e940eedd30ceb2b716a54a3d5dc@users.noreply.github.com>
ros2/rclpy,ros2/rclpy