openflow_input: fix naming of group type in group desc stats entry

The C backend blacklists the name "type" so accessors aren't generated.
"group_type" works and is consistent with the other classes.
diff --git a/openflow_input/standard-1.1 b/openflow_input/standard-1.1
index 85f696f..f985b6d 100644
--- a/openflow_input/standard-1.1
+++ b/openflow_input/standard-1.1
@@ -1202,7 +1202,7 @@
 
 struct of_group_desc_stats_entry {
     uint16_t length;
-    uint8_t type;
+    uint8_t group_type;
     pad(1);
     uint32_t group_id;
     list(of_bucket_t) buckets;