openflow_input: add discriminator to of_table_feature_prop_experimenter

The standard-1.4 input files already contain this discriminator, so the IR
failed to unify.
diff --git a/openflow_input/standard-1.3 b/openflow_input/standard-1.3
index 64a465d..3a627e1 100644
--- a/openflow_input/standard-1.3
+++ b/openflow_input/standard-1.3
@@ -1700,7 +1700,7 @@
 struct of_table_feature_prop_experimenter : of_table_feature_prop {
     uint16_t         type == 65534;
     uint16_t         length;
-    uint32_t         experimenter;
+    uint32_t         experimenter == ?;
     uint32_t         subtype;
     of_octets_t      experimenter_data;
 };
@@ -1708,7 +1708,7 @@
 struct of_table_feature_prop_experimenter_miss : of_table_feature_prop {
     uint16_t         type == 65535;
     uint16_t         length;
-    uint32_t         experimenter;
+    uint32_t         experimenter == ?;
     uint32_t         subtype;
     of_octets_t      experimenter_data;
 };