netfilter: xtables: Bring SPDX identifier back

Commit e2be04c7f995 ("License cleanup: add SPDX license identifier to
uapi header files with a license") added the correct SPDX identifier to
include/uapi/linux/netfilter/xt_IDLETIMER.h.

A subsequent commit removed it for no reason and reintroduced the UAPI
license incorrectness as the file is now missing the UAPI exception
again.

Add it back and remove the GPLv2 boilerplate while at it.

Fixes: 68983a354a65 ("netfilter: xtables: Add snapshot of hardidletimer target")
Cc: Manoj Basapathi <[e-mail]>
Cc: Subash Abhinov Kasiviswanathan <[e-mail]>
Cc: Pablo Neira Ayuso <[e-mail]>
Cc: [e-mail]
Signed-off-by: Thomas Gleixner <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

include/uapi/linux/netfilter/xt_IDLETIMER.h

···
+/* SPDX-License-Identifier: GPL-2.0-only WITH Linux-syscall-note */
 /*
- * linux/include/linux/netfilter/xt_IDLETIMER.h
- *
  * Header file for Xtables timer target module.
  *
  * Copyright (C) 2004, 2010 Nokia Corporation
···
  * by Luciano Coelho <[e-mail]>
  *
  * Contact: Luciano Coelho <[e-mail]>
- *
- * This program is free software; you can redistribute it and/or
- * modify it under the terms of the GNU General Public License
- * version 2 as published by the Free Software Foundation.
- *
- * This program is distributed in the hope that it will be useful, but
- * WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
- * General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program; if not, write to the Free Software
- * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
- * 02110-1301 USA
  */
 
 #ifndef _XT_IDLETIMER_H