gsm0808: add function to convert AMR S15-S0 to gsm0408 settings

Add a function to convert S0-S15 bits to struct gsm48_multi_rate_conf,
which hold the codec settings for AMR.

Change-Id: I973736273c236eee84decf15868190e339c3fed4
Related: OS#3548
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 3d08232..dcc491d 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -206,6 +206,7 @@
 gsm0808_chan_type_to_speech_codec;
 gsm0808_speech_codec_from_chan_type;
 gsm0808_sc_cfg_from_gsm48_mr_cfg;
+gsm48_mr_cfg_from_gsm0808_sc_cfg;
 gsm0808_speech_codec_type_names;
 gsm0808_permitted_speech_names;
 gsm0808_chosen_enc_alg_names;