windows: Fix WinUSB isochronous transfer regression

Put back missing CHECK_WINUSBX_AVAILABLE macro.

The subapi is generally set in the backend functions themselves, using
the macro CHECK_WINUSBX_AVAILABLE. However, this macro was (likely
inadvertently) removed from winusbx_copy_transfer_data() in commit
9c28ad2 causing the subapi in this function to always be SUB_API_NOTSET.
This results in the error:

  libusb: debug [winusbx_copy_transfer_data] unsupported API call for 'copy_transfer_data' (unrecognized device driver)
References #749
Closes #1095
diff --git a/libusb/os/windows_winusb.c b/libusb/os/windows_winusb.c
index 1303707..fed7d0d 100644
--- a/libusb/os/windows_winusb.c
+++ b/libusb/os/windows_winusb.c
@@ -3229,6 +3229,9 @@
 	int i;
 	if (transfer->type == LIBUSB_TRANSFER_TYPE_ISOCHRONOUS) {
+		struct winusb_device_priv *priv = usbi_get_device_priv(transfer->dev_handle->dev);
 		// for isochronous, need to copy the individual iso packet actual_lengths and statuses
 		if ((sub_api == SUB_API_LIBUSBK) || (sub_api == SUB_API_LIBUSB0)) {
 			// iso only supported on libusbk-based backends for now
diff --git a/libusb/version_nano.h b/libusb/version_nano.h
index 26ed7e7..78fef60 100644
--- a/libusb/version_nano.h
+++ b/libusb/version_nano.h
@@ -1 +1 @@
-#define LIBUSB_NANO 11712
+#define LIBUSB_NANO 11713