treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 135

Based on 1 normalized pattern(s):

  this source code is free software you can redistribute it and or
  modify it under the terms of the gnu general public license as
  published by the free software foundation either version 2 of the
  license or at your option any later version

extracted by the scancode license scanner the SPDX license identifier

  GPL-2.0-or-later

has been chosen to replace the boilerplate/reference in 1 file(s).
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org>
Reviewed-by: Allison Randal <allison@lohutok.net>
Reviewed-by: Richard Fontana <rfontana@redhat.com>
Cc: linux-spdx@vger.kernel.org
Link: https://lkml.kernel.org/r/20190524100843.782742400@linutronix.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/xen/preempt.c

+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Preemptible hypercalls
  *
  * Copyright (C) 2014 Citrix Systems R&D ltd.
- *
- * This source code is free software; you can redistribute it and/or
- * modify it under the terms of the GNU General Public License as
- * published by the Free Software Foundation; either version 2 of the
- * License, or (at your option) any later version.
  */
 
 #include <linux/sched.h>