usb: fix host-stub.c build race

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20171004125210.7817-1-kraxel@redhat.com
