Use common FindTinyXML.cmake module

[wjwwood]

There is a new package called cmake_modules, which we just created:

https://github.com/ros/cmake_modules

It provides a common FindTinyXML.cmake CMake module, which we should all try to use in order to prevent issues when doing things like cross-compiling.

This is a low priority, but a good idea. The package is available for Hydro and Groovy, it will be in public on the next sync.