use const auto & to iterate over parameters (#1593)

Clang is complaining about the looping variable being referenced as `const val` but should rather be `const ref`.

```
/Users/karsten/workspace/ros2/ros2_master/src/ros2/rclcpp/rclcpp/src/rclcpp/parameter_service.cpp:46:25: error: loop variable 'param' of type 'const rclcpp::Parameter' creates a copy from type 'const rclcpp::Parameter' [-Werror,-Wrange-loop-construct]
        for (const auto param : parameters) {
                        ^
/Users/karsten/workspace/ros2/ros2_master/src/ros2/rclcpp/rclcpp/src/rclcpp/parameter_service.cpp:46:14: note: use reference type 'const rclcpp::Parameter &' to prevent copying
        for (const auto param : parameters) {
             ^~~~~~~~~~~~~~~~~~
                        &
1 error generated.
```

Signed-off-by: Karsten Knese <Karsten1987@users.noreply.github.com>

rclcpp/src/rclcpp/parameter_service.cpp

@@ -43,7 +43,7 @@ ParameterService::ParameterService(
     {
       try {
         auto parameters = node_params->get_parameters(request->names);
-        for (const auto param : parameters) {
+        for (const auto & param : parameters) {
           response->values.push_back(param.get_value_message());
         }
       } catch (const rclcpp::exceptions::ParameterNotDeclaredException & ex) {