RSL: add code to generate SACCH INFO MODIFY command

SACCH INFO MODIFY is intended to modify the SACCH content for a single
logical channel (as opposed to the default SACCH FILLING)
diff --git a/openbsc/include/openbsc/abis_rsl.h b/openbsc/include/openbsc/abis_rsl.h
index f95fa31..295b01f 100644
--- a/openbsc/include/openbsc/abis_rsl.h
+++ b/openbsc/include/openbsc/abis_rsl.h
@@ -85,6 +85,9 @@
 int rsl_ccch_conf_to_bs_ccch_sdcch_comb(int ccch_conf);
 int rsl_number_of_paging_subchannels(struct gsm_bts *bts);
 
+int rsl_sacch_info_modify(struct gsm_lchan *lchan, u_int8_t type,
+			  const u_int8_t *data, int len);
+
 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);