openflow_input: fix of_group_desc_stats_entry.group_type

use enum ofp_group_type for consistency with the group_mod messages
diff --git a/openflow_input/standard-1.3 b/openflow_input/standard-1.3
index 4d0ca7e..1518931 100644
--- a/openflow_input/standard-1.3
+++ b/openflow_input/standard-1.3
@@ -1427,7 +1427,7 @@
 
 struct of_group_desc_stats_entry {
     uint16_t length;
-    uint8_t group_type;
+    enum ofp_group_type group_type;
     pad(1);
     uint32_t group_id;
     list(of_bucket_t) buckets;