Added another place to search for libs in the FindSFML CMake module

By default, when you build SFML in Visual Studio, it puts the .libs into SFML/lib/Debug (if you built it in Debug mode) or SFML/lib/Release (if you built it in Release mode).  The FindSFML CMake module does not check for these folders, so it will fail unless you move all of these files into SFML/lib.