You should always declare dependencies on other message packages here. This happens to work because `std_msgs`'s types like Header can be reference without a scope, as opposed to other message packages, e.g. `geometry_msgs`'s `Point` message is referenced as `geometry_msgs/Point`:

https://github.com/ros/common_msgs/blob/groovy-devel/visualization_msgs/msg/ImageMarker.msg#L15