Fix transmission interface tinyxml build error

Second try, based on https://github.com/ros-controls/ros_control/pull/92

Fix for:
```
/tmp/buildd/ros-hydro-transmission-interface-0.5.0-0precise-20130719-0615/include/transmission_interface/transmission_info.h:48:21: fatal error: tinyxml.h: No such file or directory
```