[Brillo] Add XmlRpc++ library Adding a third-party native XML RPC library for Shill testing on Brillo. Bug: 24335496 TEST: source build/envsetup.sh && lunch aosp_shamu-userdebug && make -j 64 Change-Id: I7a4d86fc3ab3b85f981e1814c1ef44eab85471b5