loci: remove header classes

Now that e.g. of_oxm_t is a valid typedef for of_object_t, there's no need for
of_oxm_header_t.
diff --git a/c_gen/match.py b/c_gen/match.py
index d6b43fa..c6459d4 100644
--- a/c_gen/match.py
+++ b/c_gen/match.py
@@ -197,8 +197,7 @@
     # Generate list of OXM names from the unified classes
     oxm_names = [x[7:] for x in of_g.unified.keys() if
                  x.startswith('of_oxm_') and
-                 x.find('masked') < 0 and
-                 x.find('header') < 0]
+                 x.find('masked') < 0]
 
     # Check that all OXMs are in the match members
     for key in oxm_names: