gsm0808: Add utils for Channel Type

The planned support for true A over IP requires the encoding of
the a Channel Type element (see also ASSIGNMENT REQUEST).

This commt adds encoding/decoding functionality and tests for
the element mentioned above, however, it is not yet actively used.

Change-Id: Id0e2164d84b8cbcc6fe6a090fc7f40a1251421d7
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 518a5aa..2ba0c42 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -146,6 +146,8 @@
 gsm0808_dec_speech_codec;
 gsm0808_enc_speech_codec_list;
 gsm0808_dec_speech_codec_list;
+gsm0808_enc_channel_type;
+gsm0808_dec_channel_type;
 
 gsm0858_rsl_ul_meas_enc;