add gsm48_pdisc_msgtype_name()

Composing the message type string requires knowing the protocol discriminator.
To ease printing the message type, add this function to switch between the
defined value_string[]s depending on pdisc.

Also publish the message type value_string[]s -- without inline functions to
access them because it is anyway more convenient to use
gsm48_pdisc_msgtype_name() instead.

Since gsm48_pdisc_msgtype_name() is nontrivial, do not add as inline function
-- in case the message type is not known, it needs a static string buffer.

Change-Id: I0fca8e95ed5c2148b1a7440eff3fc9c7583898df
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 60f83de..4a33c46 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -218,6 +218,10 @@
 gsm48_chan_mode_names;
 gsm_chan_t_names;
 gsm48_pdisc_names;
+gsm48_rr_msgtype_names;
+gsm48_mm_msgtype_names;
+gsm48_cc_msgtype_names;
+gsm48_pdisc_msgtype_name;
 
 gsm_7bit_decode;
 gsm_7bit_decode_ussd;