staging: goldfish: Add SPDX license identifier.

Fix checkpatch warning by adding the SPDX license identifier.

---
 drivers/staging/goldfish/goldfish_audio.c | 1 +
 1 file changed, 1 insertion(+)

index 3a75df1d2a0a..d4520490cf6d 100644
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * drivers/misc/goldfish_audio.c
  *

2.17.1