snmp: set unidirectional transaction flag

rust/src/snmp/snmp.rs

@@ -584,7 +584,7 @@ pub unsafe extern "C" fn rs_register_snmp_parser() {
         get_tx_iterator    : None,
         get_tx_data        : rs_snmp_get_tx_data,
         apply_tx_config    : None,
-        flags              : 0,
+        flags              : APP_LAYER_PARSER_OPT_UNIDIR_TXS,
         truncate           : None,
     };
     let ip_proto_str = CString::new("udp").unwrap();