Add function to encode classmark

The code is based on Osmocom-BB implementation.

Change-Id: I78f6968edaa3ed535673411fb2a80060a472290f
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 7a74718..b6152ce 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -245,6 +245,7 @@
 gsm48_encode_signal;
 gsm48_encode_ssversion;
 gsm48_encode_useruser;
+gsm48_encode_classmark1;
 gsm48_generate_lai;
 gsm48_generate_mid;
 gsm48_generate_mid_from_imsi;