nat: Set the MGCP audio payload to -1.

This value is copied into the bts_audio_payload when allocating
a BTS MGCP endpoint. For the nat we have actually no interest in
patching MGCP messages. We will patch them to the network because
the code will do it anyway, we will not patch things back to the
BTS.

openbsc/src/nat/bsc_mgcp_utils.c

@@ -443,6 +443,7 @@ int bsc_mgcp_init(struct bsc_nat *nat)
 	}
 
 	/* some more MGCP config handling */
+	nat->mgcp_cfg->audio_payload = -1;
 	nat->mgcp_cfg->data = nat;
 	nat->mgcp_cfg->policy_cb = bsc_mgcp_policy_cb;
 	nat->bsc_endpoints = talloc_zero_array(nat,