Add support for AMR frames to MNCC/RTP interface

AMR rate is currently fixed to 5.9k.
diff --git a/openbsc/src/libmsc/gsm_04_08.c b/openbsc/src/libmsc/gsm_04_08.c
index 74dc27f..58f97f4 100644
--- a/openbsc/src/libmsc/gsm_04_08.c
+++ b/openbsc/src/libmsc/gsm_04_08.c
@@ -2965,6 +2965,7 @@
 	case GSM_TCHF_FRAME:
 	case GSM_TCHF_FRAME_EFR:
 	case GSM_TCHH_FRAME:
+	case GSM_TCH_FRAME_AMR:
 		/* Find callref */
 		trans = trans_find_by_callref(net, data->callref);
 		if (!trans) {