Attempt to fix transmission interface tinyxml build error

I'm not sure if this fixes the issue or not, I could not replicate the build failures on my computer.

Currently Jenkins says:
```
/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
```
@adolfo-rt