This function is only available in the headers if unconditionally
targeting a version >= XP. It is not strictly necessary, since we
try loading these functions dynamically at runtime on windows in
the fallback, but this makes things a bit faster and more
straightforward.

On mingw32, this function isn't visible by default, while it is
on mingw64 (on both 32 and 64 bit).
Signed-off-by: Martin Storsjö <martin@martin.st>