gsm0808: Add utils for Speech Codec List and Speech Codec

The planned support for true A over IP requires the encoding and
decoding of a so called "Speech Codec Element" element.

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

Change-Id: I0e1e2edf47adaa45b22d4b0bcae3640dba7ca200
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 3ad847d..c89cbe4 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -139,6 +139,10 @@
 gsm0808_prepend_dtap_header;
 gsm0808_enc_aoip_trasp_addr;
 gsm0808_dec_aoip_trasp_addr;
+gsm0808_enc_speech_codec;
+gsm0808_dec_speech_codec;
+gsm0808_enc_speech_codec_list;
+gsm0808_dec_speech_codec_list;
 
 gsm0858_rsl_ul_meas_enc;