Search for urdfdom_headers when USE_EXTERNAL_URDF=True

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/72)) by Steve Peters (Bitbucket: [Steven Peters](https://bitbucket.org/%7B2ccfed09-18b8-4921-8d58-15ef01092802%7D/), GitHub: [scpeters](https://github.com/scpeters)).**

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Currently we search for urdfdom in [cmake/SearchForStuff.cmake](https://github.com/osrf/sdformat/blob/e50e07defeca1ebde8fa152006f7e45a26d8fcd8/cmake/SearchForStuff.cmake#L51), but we do not search for `urdfdom_headers`. I only noticed this when doing a `catkin_make_isolated` devel build (which installs `urdfdom_headers` to a different prefix) and was surprised that it couldn't find the header files.