gsm_12_21.h: Add header description pointing to TS files

Change-Id: I20d5bb659d84228a951b66228b7c172bce3ee929
diff --git a/include/osmocom/gsm/protocol/gsm_12_21.h b/include/osmocom/gsm/protocol/gsm_12_21.h
index 4baa7e8..96a6f51 100644
--- a/include/osmocom/gsm/protocol/gsm_12_21.h
+++ b/include/osmocom/gsm/protocol/gsm_12_21.h
@@ -1,3 +1,4 @@
+/* 3GPP TS 12.21, nowadays 3GPP TS 52.021 */
 /*
  * (C) 2008-2009 by Harald Welte <laforge@gnumonks.org>
  * All Rights Reserved