mgcp: Allow to disable transcoding for trunks

We might have compiled transcoding into the MGW but
we don't want to enable it for a given user. Add a new
switch that should allow that.

I had manually tested the allow-transcoding/no allow
VTY interface for the primary interface and a new trunk
using show running-config.
diff --git a/openbsc/src/libmgcp/mgcp_transcode.c b/openbsc/src/libmgcp/mgcp_transcode.c
index 04fb2dc..daf2510 100644
--- a/openbsc/src/libmgcp/mgcp_transcode.c
+++ b/openbsc/src/libmgcp/mgcp_transcode.c
@@ -153,6 +153,13 @@
 	if (!src_end)
 		return 0;
 
+	if (endp->tcfg->no_audio_transcoding) {
+		LOGP(DMGCP, LOGL_NOTICE,
+			"Transcoding disabled on endpoint 0x%x\n",
+			ENDPOINT_NUMBER(endp));
+		return 0;
+	}
+
 	src_fmt = get_audio_format(src_codec);
 	dst_fmt = get_audio_format(dst_codec);