cryptui: add CryptUIWizDigitalSign stub

[Vincent Povirk]

This was surprising to me as well, but apparently we are using long
for HWND in spec files.

It'd be nice to hear about why we do this (and why it's safe for us to
do this) when HWND is a pointer-sized type, instead of a repeated
assertion of "this is wrong", when it was a reasonable thing for
someone unaware of this rule to expect. I have some guesses, but I'd
rather hear from someone who knows about it.