gprs_ns: Allow to set the DSCP for the UDP socket.

Allow to tag the NS service with a custom DSCP.
diff --git a/src/gb/gprs_ns.c b/src/gb/gprs_ns.c
index cdcf36e..ef937d9 100644
--- a/src/gb/gprs_ns.c
+++ b/src/gb/gprs_ns.c
@@ -1051,6 +1051,13 @@
 	if (ret < 0)
 		return ret;
 
+	ret = setsockopt(nsi->nsip.fd.fd, IPPROTO_IP, IP_TOS,
+				&nsi->nsip.dscp, sizeof(nsi->nsip.dscp));
+	if (ret < 0)
+		LOGP(DNS, LOGL_ERROR,
+			"Failed to set the DSCP to %d with ret(%d) errno(%d)\n",
+			nsi->nsip.dscp, ret, errno);
+
 
 	return ret;
 }