"std_msgs/Header header" is not considered to be a header  in messages generated with rosbuild

Follow up of ros/gencpp#21.

The message parser used for `rosbuild` only considers the field `Header header` to be a header.

`std_msgs/Header header` is not accepted which leads to not generating certain message traits: https://github.com/ros/ros/blob/01f9fc1140e2940be2799688aca85d5441807fa7/core/roslib/src/roslib/msgs.py#L304