x86/jailhouse: Fix incorrect SPDX identifier

GPL2.0 is not a valid SPDX identiier. Replace it with GPL-2.0.

Fixes: 4a362601ba ("x86/jailhouse: Add infrastructure for running in non-root cell")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Link: https://lkml.kernel.org/r/20180422220832.815346488@linutronix.de

arch/x86/kernel/jailhouse.c

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL2.0
+// SPDX-License-Identifier: GPL-2.0
 /*
  * Jailhouse paravirt_ops implementation
  *