add gsm48_pdisc_names and gsm48_pdisc_name()

I often want to log the protocol discriminator in the openbsc debug log. It's
more useful to get the name directly instead of looking it up every time.

Change-Id: I0f053e2a4360b27ffccda7cf82469fb1b1cbb3ae
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 8d28476..60f83de 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -217,6 +217,7 @@
 gsm48_mcc_mnc_from_bcd;
 gsm48_chan_mode_names;
 gsm_chan_t_names;
+gsm48_pdisc_names;
 
 gsm_7bit_decode;
 gsm_7bit_decode_ussd;