add DSS logging category

Change-Id: Id7e04c9f5088334cd5ec6cfdb6a9b3a2a7e7fda0
diff --git a/include/osmocom/msc/transaction.h b/include/osmocom/msc/transaction.h
index 6b82390..c63ef5b 100644
--- a/include/osmocom/msc/transaction.h
+++ b/include/osmocom/msc/transaction.h
@@ -168,10 +168,7 @@
 	case TRANS_SMS:
 		return DLSMS;
 	case TRANS_USSD:
-		/* FIXME: traditionally (before LOG_TRANS() was added in I2e60964d7a3c06d051debd1c707051a0eb3101ba /
-		 * ff7074a0c7b62025473d8f1a950905ac2cb2f31c), all USSD logging happened on DMM. Instead, it probably
-		 * deserves its own logging subsystem. */
-		return DMM;
+		return DSS;
 	default:
 		break;
 	}