mgcp: Honor the rtp IP_TOS settings for Osmux

Honor the IP_TOS settings for Osmux as well. Re-use the RTP
setting as it makes sense to classify the audio packets the
same way.

Fixes: OW#1369
diff --git a/openbsc/src/libmgcp/mgcp_osmux.c b/openbsc/src/libmgcp/mgcp_osmux.c
index d5e671d..7f61173 100644
--- a/openbsc/src/libmgcp/mgcp_osmux.c
+++ b/openbsc/src/libmgcp/mgcp_osmux.c
@@ -420,6 +420,7 @@
 		LOGP(DMGCP, LOGL_ERROR, "cannot bind OSMUX socket\n");
 		return ret;
 	}
+	mgcp_set_ip_tos(osmux_fd.fd, cfg->endp_dscp);
 	osmux_fd.when |= BSC_FD_READ;
 
 	ret = osmo_fd_register(&osmux_fd);