rsl: Add method to send a SMSCB inside a SMS BROADCAST COMMAND

* Untested code for sending the SMS BROADCAST COMMAND.
diff --git a/openbsc/include/openbsc/abis_rsl.h b/openbsc/include/openbsc/abis_rsl.h
index e87f430..4a33410 100644
--- a/openbsc/include/openbsc/abis_rsl.h
+++ b/openbsc/include/openbsc/abis_rsl.h
@@ -84,5 +84,9 @@
 int rsl_chan_bs_power_ctrl(struct gsm_lchan *lchan, unsigned int fpc, int db);
 int rsl_chan_ms_power_ctrl(struct gsm_lchan *lchan, unsigned int fpc, int dbm);
 
+/* SMSCB functionality */
+int rsl_sms_cb_command(struct gsm_bts *bts, uint8_t chan_number,
+		       uint8_t cb_command, const uint8_t *data, int len);
+
 #endif /* RSL_MT_H */