sms: Increment the RP Message Reference for each transaction

Each RP-DATA should have a unique msg reference. Currently 42 is
used for all of these. Remember the last reference we used and
increment it on the next SMS. Do not track if the reference is
still in use a clash is a lot less likely now. First unless SMPP
is used only one SMS is delivered at a time, second the transaction
space is a lot smaller than the one for the reference.
diff --git a/openbsc/include/openbsc/gsm_04_11.h b/openbsc/include/openbsc/gsm_04_11.h
index 72b6948..00c3a19 100644
--- a/openbsc/include/openbsc/gsm_04_11.h
+++ b/openbsc/include/openbsc/gsm_04_11.h
@@ -37,4 +37,6 @@
 int gsm411_send_sms(struct gsm_subscriber_connection *conn,
 		    struct gsm_sms *sms);
 void gsm411_sapi_n_reject(struct gsm_subscriber_connection *conn);
+
+uint8_t sms_next_rp_msg_ref(struct gsm_subscriber_connection *conn);
 #endif