media: dvb_vb2: add SPDX headers

This code is released under GPL. Add the corresponding SPDX
headers.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/dvb-core/dvb_vb2.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * dvb-vb2.c - dvb-vb2
  *

drivers/media/dvb-core/dvb_vb2.h

 /*
+ * SPDX-License-Identifier: GPL-2.0
+ *
  * dvb-vb2.h - DVB driver helper framework for streaming I/O
  *
  * Copyright (C) 2015 Samsung Electronics