staging: gasket: add SPDX identifiers to all files.

It's good to have SPDX identifiers in all files to make it easier to
audit the kernel tree for correct licenses.

Fix up the all of the staging gasket files to have a proper SPDX
identifier, based on the license text in the file itself.  The SPDX
identifier is a legally binding shorthand, which can be used instead of
the full boiler plate text.

Cc: Rob Springer <rspringer@google.com>
Cc: John Joseph <jnjoseph@google.com>
Cc: Ben Chan <benchan@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

gasket_sysfs.c

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0
 /* Copyright (C) 2018 Google, Inc.
  *
  * This software is licensed under the terms of the GNU General Public