wtsapi32: Make WTSRegisterSessionNotificationEx's return consistent with WTSRegisterSessionNotification.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=47433Signed-off-by: Patrick Hibbs <hibbsncc1701@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>