Update internal GPRS cipher API

Update internal API (for GPRS cipher implementors): make it compliant
with ETSI TS 155.22. External API left untouched.
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 6886a6c..a526756 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -40,6 +40,7 @@
 gprs_cipher_register;
 gprs_cipher_run;
 gprs_cipher_supported;
+gprs_cipher_key_length;
 gprs_tlli_type;
 gprs_tmsi2tlli;