smscb: Fix adding of SMSCB messages when no message with lower period exists

Adding SMSCB messages to a BTS so far only worked if there were
existing messages with a lower scheduling period than the new message.

Before this patch, it fails for new messages if they are of equal or
lower scheduling period than the existing messages.

Change-Id: I69a05b22200b3a1ee406b0673553e135603d723b

src/osmo-bsc/smscb.c

@@ -110,6 +110,8 @@ static void __bts_smscb_add(struct bts_smscb_chan_state *cstate, struct bts_smsc
 			return;
 		}
 	}
+	/* we didn't find any messages with longer period than us, insert us at tail */
+	llist_add_tail(&new->list, &cstate->messages);
 }
 
 /* stringify a SMSCB for logging */