Added OpenAL and SDL Windows dependency DLLs to mirrors

This fixes https://github.com/OpenRA/OpenRA/wiki/FAQ#the-downloads-during-windows-installation-and-first-game-start-dont-work as it is a common annoyance that the SDL/OpenAL webserver went down when somebody tried to run the OpenRA Setup.exe only this time it happened during the first minutes after the stable release announcement when I tried to prepare the patch for Desura which required a clean install so I noticed when a file got removed in my proposed patch there and could help people immediately. http://www.libsdl.org/ was online again after a short while so this is not urgent.