openflow_input: use of_ prefix in input files

No change to generated code.
diff --git a/openflow_input/nicira_dec_ttl b/openflow_input/nicira_dec_ttl
index b507d57..6d113fb 100644
--- a/openflow_input/nicira_dec_ttl
+++ b/openflow_input/nicira_dec_ttl
@@ -27,7 +27,7 @@
 
 #version any
 
-struct ofp_action_nicira_dec_ttl {
+struct of_action_nicira_dec_ttl {
     uint16_t type;      // OF_ACTION_TYPE_EXPERIMENTER
     uint16_t len;
     uint32_t experimenter;    // OF_EXPERIMENTER_ID_NICIRA