I am not sure I can explain the exact reason for that being disallowed, but I can tell you how to fix it:
Now I'll try to explain it anyway. The compiler tries to figure out the type of what you are passing to std::list<int>::sort, and it can't figure it out because you are passing something with an "unresolved overloaded function type" (that's what the error message in g++ calls it). If you specify sortDescending<int>, now the compiler knows the type being passed and can continue to figure out what version of the overloaded std::list<int>::sort you are calling.
EDIT: Double ninja'd. I must be getting slow.