lightnvm: pblk: add SPDX license tag

Add GLP-2.0 SPDX license tag to all pblk files

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/drivers/lightnvm/pblk.h b/drivers/lightnvm/pblk.h
index 4c015c4..e710f6d 100644
--- a/drivers/lightnvm/pblk.h
+++ b/drivers/lightnvm/pblk.h
@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Copyright (C) 2015 IT University of Copenhagen (rrpc.h)
  * Copyright (C) 2016 CNEX Labs