Windows: opt into the libusb UsbDk backend at init (parity with libfreenect2)

[Le-Syl21]

Summary

Mirror libfreenect2's Windows init behaviour by calling
libusb_set_option(ctx, LIBUSB_OPTION_USE_USBDK) immediately after
libusb_init in fnusb_init, guarded by #ifdef _WIN32. This is the
exact same one-line opt-in libfreenect2 already does in
libfreenect2.cpp:

```c
#if defined(WIN32) || defined (WIN32) || defined(WINDOWS)
(void)libusb_set_option(usb_context, LIBUSB_OPTION_USE_USBDK);
#endif
```

Why

Without the opt-in, libusb on Windows uses the WinUSB backend by
default. WinUSB can only attach to devices whose function driver is
WinUSB / libusbK / libusb-win32. On a typical user setup, the Kinect
v1 sub-devices (`Xbox NUI Motor`, `Xbox NUI Audio`, `Xbox NUI
Camera`) are either:

• bound to no driver (Windows shows them as Unknown device) → libusb
  cannot enumerate them at all, or
• bound to the Microsoft Kinect SDK driver → libusb enumerates them
  but `libusb_open` returns `LIBUSB_ERROR_NOT_SUPPORTED` (-12).

Today the README's only documented fix for this is "run Zadig, pick
each of the three sub-devices, replace their driver with libusbK".
That works but is invasive: it removes the Microsoft SDK driver, has
to be redone if the user later reinstalls the SDK, and requires three
manual Zadig replacements per Kinect.

UsbDk solves the same problem in a much friendlier way — it's a filter
driver shipped as a signed MSI by Daynix that slots above the existing
function driver and gives userspace tools libusb-compatible access to
any USB device. libfreenect2 already takes advantage of it via the
opt-in above; this PR brings libfreenect to parity.

After this patch, a Windows user can install
UsbDk once and have
libfreenect open the Kinect regardless of whether the Microsoft Kinect
SDK driver is installed — no per-device Zadig steps, no driver
replacement.

Verified

• Identical guard pattern to libfreenect2 (`_WIN32 || WIN32 ||
  WINDOWS`).
• Return value cast to `(void)` exactly like libfreenect2 — the call
  only ever fails on non-Windows platforms (where it's compiled out).
• Zero behaviour change on Linux and macOS (preprocessor-gated).
• `LIBUSB_OPTION_USE_USBDK` was added in libusb 1.0.22 (Aug 2018).
  libfreenect's README already recommends libusb ≥ 1.0.22 for native
  libusbK isochronous transfers on Windows, so no new minimum-version
  requirement is introduced for users actually building the Windows
  target.

Test plan

• Builds cleanly on Linux (no Windows code path touched).
• Behavioural test on Windows with UsbDk installed and the
  Microsoft Kinect SDK driver attached — `freenect_open_device`
  should now succeed where it previously returned -12.