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

Allow to tag the NS service with a custom DSCP.
diff --git a/include/osmocom/gprs/gprs_ns.h b/include/osmocom/gprs/gprs_ns.h
index a7f32b2..8fca70c 100644
--- a/include/osmocom/gprs/gprs_ns.h
+++ b/include/osmocom/gprs/gprs_ns.h
@@ -73,6 +73,7 @@
 		struct osmo_fd fd;
 		uint32_t local_ip;
 		uint16_t local_port;
+		int dscp;
 	} nsip;
 	/*! \brief NS-over-FR-over-GRE-over-IP specific bits */
 	struct {