staging: xgifb: add SPDX identifiers to the remaining holdout file

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

Update the xgifb/XGI_main_26.c file with the correct SPDX license
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.

This work is based on a script and data from Thomas Gleixner, Philippe
Ombredanne, and Kate Stewart.

Cc: Arnaud Patard <arnaud.patard@rtp-net.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/xgifb/XGI_main_26.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * XG20, XG21, XG40, XG42 frame buffer device
  * for Linux kernels  2.5.x, 2.6.x