signal.h: make om2k mo const

The om2k mo that is put into the nsd as reference to notify other
entities about the signal change can be const. Its only accessed
read-only (if at all) and also the API in abis_om2000.h suggests that
the om2k mo should be const.

Change-Id: Id0969d44855506af18974de1ea81105653920d2f
Related: OS#5634

include/osmocom/bsc/signal.h

@@ -138,7 +138,7 @@ struct nm_statechg_signal_data {
 	/* This pointer is valid for TS 12.21 MO */
 	struct abis_om_obj_inst *obj_inst;
 	/* This pointer is valid for RBS2000 MO */
-	struct abis_om2k_mo *om2k_mo;
+	const struct abis_om2k_mo *om2k_mo;
 };
 
 /* data for <SS_NM, S_NM_RUNNING_CHG>: */