Fix issue with build ordering

See:

https://github.com/ros/catkin/issues/435
http://answers.ros.org/question/64448/trouble-with-catkin-compiling-node-and-generating-messages-in-same-package/

It turns out that the error was actually occurring in the `ublox_gps` package and not the `ublox_msgs` package. This is fixed by adding a target dependency on `ublox_msgs`'s exported targets in `ublox_gps`. Note that the `<package>_EXPORTED_TARGETS` variable is only available in `genmsg` 0.4.18 and on.