windows: Mark libusb_set_option as WINAPIV

WINAPI evaluates to __stdcall, which does not work with variadic
functions. MSVC and MinGW implicitly demote this to __cdecl (clang does
too and also warns about it). Instead of having this implicit, make it
explicit.

Closes #1160

Signed-off-by: Rosen Penev <rosenp@gmail.com>