missing add_dependency macro

[tkruse]

According to http://answers.ros.org/question/53265/catkin-messages-and-node-in-same-package

targets xyz that depend on c++ message headers need to declare a dependency to the genmsg target xyz_gencpp

Affects http://www.ros.org/wiki/ROS/Tutorials/WritingPublisherSubscriber%28c%2B%2B%29