HID: wacom: re-add accidentally dropped Lenovo PID

Dropped in the following commit:

commit a3e6f654 ("Input: wacom - keep wacom_ids ordered")
Reported-by: Hans Spath <inbox-546@hans-spath.de>
CC: stable@vger.kernel.org    # v3.17+
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/wacom_wac.c

@@ -3026,6 +3026,7 @@ const struct hid_device_id wacom_ids[] = {
 	{ USB_DEVICE_WACOM(0x4004) },
 	{ USB_DEVICE_WACOM(0x5000) },
 	{ USB_DEVICE_WACOM(0x5002) },
+	{ USB_DEVICE_LENOVO(0x6004) },
 
 	{ USB_DEVICE_WACOM(HID_ANY_ID) },
 	{ }