gsm0808: add function gsm0808_chan_type_to_speech_code()

The permitted speech field used in channel type element (struct
gsm0808_channel_type) uses a different representation as
the type field in the speech codec element (struct
gsm0808_speech_codec)

This patch adds a function to convert from permitted speech to
speech codec type.

Change-Id: Ib26a9c20864459b2baaa04f49b6e7902ba44b7cb
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 9613c4e..ccc5b9a 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -162,6 +162,7 @@
 gsm0808_dec_encrypt_info;
 gsm0808_enc_cell_id_list;
 gsm0808_dec_cell_id_list;
+gsm0808_convert_to_speech_codec_type;
 
 gsm0858_rsl_ul_meas_enc;