loci: do not generate header classes for messages

These conflict with the real classes of the same name.
diff --git a/c_gen/codegen.py b/c_gen/codegen.py
index fe5e2ad..89a0a81 100644
--- a/c_gen/codegen.py
+++ b/c_gen/codegen.py
@@ -122,7 +122,7 @@
 # TODO remove header classes and use the corresponding class instead
 def generate_header_classes(install_dir):
     for cls in of_g.standard_class_order:
-        if cls.find("_header") < 0:
+        if cls.find("_header") < 0 or cls in ["of_header", "of_bsn_header", "of_nicira_header"]:
             continue
         with template_utils.open_output(install_dir, "loci/src/%s.c" % cls) as out:
             util.render_template(out, "class.c",