loci: OF_HEADER is a message

It's the base class for all messages.
diff --git a/c_gen/loxi_utils_legacy.py b/c_gen/loxi_utils_legacy.py
index 220d043..48aaca0 100644
--- a/c_gen/loxi_utils_legacy.py
+++ b/c_gen/loxi_utils_legacy.py
@@ -93,7 +93,7 @@
     """
     Return True if cls is a message object based on info in unified
     """
-    return "xid" in of_g.unified[cls]["union"] and cls != "of_header"
+    return "xid" in of_g.unified[cls]["union"]
 
 def class_is_tlv16(cls):
     """