Add function to check GMM encryptability

Check if particular GMM message can be encrypted according to 3GPP TS
24.008 ยง 4.7.1.2

Related: OS#1582
Change-Id: I7ad0e03c2c738d174dd6bc3453f332eeb8da1e7d
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 19ebb5b..c68cf2a 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -124,6 +124,7 @@
 gsm48_cc_msg_name;
 gsm48_cc_state_name;
 gsm48_construct_ra;
+gsm48_hdr_gmm_cipherable;
 gsm48_decode_bcd_number;
 gsm48_decode_bearer_cap;
 gsm48_decode_called;