gsm0808: Add utils for AoIP Transport Layer Address

The planned support for true A over IP requires the encoding and
decoding of a so called "AoIP Transport Layer Address" element.

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

Change-Id: I57933b0a06a3f54ec2a41e6ecb6ced9fbbc89332
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 5649e71..3ad847d 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -137,6 +137,8 @@
 gsm0808_create_reset;
 gsm0808_create_sapi_reject;
 gsm0808_prepend_dtap_header;
+gsm0808_enc_aoip_trasp_addr;
+gsm0808_dec_aoip_trasp_addr;
 
 gsm0858_rsl_ul_meas_enc;