Loxigen: added src/dst meta id matches

Encode a 1 byte src and 1 byte dst 'meta' ID into
ofp_match's pad2 byte.
diff --git a/c_gen/c_match.py b/c_gen/c_match.py
index 84bc782..331b2c3 100644
--- a/c_gen/c_match.py
+++ b/c_gen/c_match.py
@@ -274,6 +274,8 @@
     OF_OXM_INDEX_IPV6_ND_TLL    = 33, /* Target link-layer for ND. */
     OF_OXM_INDEX_MPLS_LABEL     = 34, /* MPLS label. */
     OF_OXM_INDEX_MPLS_TC        = 35, /* MPLS TC. */
+    OF_OXM_INDEX_SRC_META_ID = 36,
+    OF_OXM_INDEX_DST_META_ID = 37,
 };
 
 #define OF_OXM_BIT(index) (((uint64_t) 1) << (index))