Fix const-ness in std::chrono::time_point construction and explicitly…

… use std::chrono::nanoseconds as std::chrono::time_point template parameter to help compilation on macOS as its std::chrono::system_clock::time_point defaults to std::chrono::milliseconds for duration type (#848)

controller_manager/src/ros2_control_node.cpp

@@ -62,8 +62,9 @@ int main(int argc, char ** argv)
 
       // for calculating sleep time
       auto const period = std::chrono::nanoseconds(1'000'000'000 / cm->get_update_rate());
-      std::chrono::system_clock::time_point next_iteration_time =
-        std::chrono::system_clock::time_point(std::chrono::nanoseconds(cm->now().nanoseconds()));
+      auto const cm_now = std::chrono::nanoseconds(cm->now().nanoseconds());
+      std::chrono::time_point<std::chrono::system_clock, std::chrono::nanoseconds>
+        next_iteration_time{cm_now};
 
       // for calculating the measured period of the loop
       rclcpp::Time previous_time = cm->now();