Fix libusb callback for Windows

Add LIBUSB_CALL so that the callback has the correct signature on
Windows (including __attribute__((stdcall))).

app/src/usb/usb.c

@@ -221,7 +221,7 @@ sc_usb_destroy(struct sc_usb *usb) {
     libusb_exit(usb->context);
 }
 
-static int
+static LIBUSB_CALL int
 sc_usb_libusb_callback(libusb_context *ctx, libusb_device *device,
                        libusb_hotplug_event event, void *userdata) {
     (void) ctx;