gsm: add gsm0502_fn2ccch_block()

This API is useful for checking whether a Downlink CCCH block belongs
to PCH or AGCH.  We need this API in osmo-bts.git and osmocom-bb.git.

Change-Id: I8cbd31226754e95887358ed83a928e2f567f4cf3
Related: OS#5500
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 1e5a7b2..609ee7e 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -111,6 +111,7 @@
 gsm0502_calc_paging_group;
 gsm0502_fn_remap;
 gsm0502_hop_seq_gen;
+gsm0502_fn2ccch_block;
 
 gsm0503_xcch;
 gsm0503_rach;