private: use dispatch_qos_class_t rather than qos_class_t

The `qos_class_t` is typealiased as `disaptch_qos_class_t` in
`dispatch/object.h` to allow portability for platforms which do not vend
the type.  Use this to allow building the new interfaces on Windows.

private/channel_private.h

@@ -259,7 +259,7 @@ dispatch_channel_create(const char *_Nullable label,
 API_AVAILABLE(macos(10.14), ios(12.0), tvos(12.0), watchos(5.0), bridgeos(4.0))
 DISPATCH_EXPORT DISPATCH_NOTHROW DISPATCH_NONNULL_ALL
 void
-dispatch_channel_wakeup(dispatch_channel_t channel, qos_class_t qos_class);
+dispatch_channel_wakeup(dispatch_channel_t channel, dispatch_qos_class_t qos_class);
 
 /*! @typedef dispatch_channel_enumerator_handler_t
  *

private/queue_private.h

@@ -537,7 +537,7 @@ SPI_AVAILABLE(macos(12.0), ios(15.0))
 DISPATCH_EXPORT DISPATCH_NONNULL1 DISPATCH_NONNULL2
 void
 dispatch_async_swift_job(dispatch_queue_t queue, void *swift_job,
-	qos_class_t qos);
+	dispatch_qos_class_t qos);
 
 __END_DECLS