openflow_input: add flags field to OF 1.3 flow stats entry

This field was padding in OF 1.2.
diff --git a/openflow_input/standard-1.3 b/openflow_input/standard-1.3
index b9c5475..b70c0e9 100644
--- a/openflow_input/standard-1.3
+++ b/openflow_input/standard-1.3
@@ -1177,7 +1177,8 @@
     uint16_t priority;
     uint16_t idle_timeout;
     uint16_t hard_timeout;
-    pad(6);
+    uint16_t flags;
+    pad(4);
     uint64_t cookie;
     uint64_t packet_count;
     uint64_t byte_count;