staging: goldfish: Add SPDX license identifier.

Fix checkpatch warning by adding the SPDX license identifier.
Signed-off-by: Rohit Sarkar <rohitsarkar5398@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/goldfish/goldfish_audio.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * drivers/misc/goldfish_audio.c
  *