ofproto: Fix build with some GCC versions.

GCC 4.8.x and possibly other versions don't like a designated initializer
for an anonymous struct, see e.g.
https://travis-ci.org/openvswitch/ovs/jobs/433747674

Fixes: f836888 ("ofproto: Handle OpenFlow version mismatch for requestforward with groups.")
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Justin Pettit <jpettit@ovn.org>

ofproto/ofproto.c

@@ -7393,13 +7393,12 @@ ofproto_group_mod_finish(struct ofproto *ofproto,
     remove_groups_postponed(&ogm->old_groups);
 
     if (req) {
-        struct ofputil_requestforward rf = {
-            .xid = req->request->xid,
-            .reason = OFPRFR_GROUP_MOD,
-            .group_mod = &ogm->gm,
-            .new_buckets = new_group ? &new_group->buckets : NULL,
-            .group_existed = group_collection_n(&ogm->old_groups) > 0,
-        };
+        struct ofputil_requestforward rf;
+        rf.xid = req->request->xid;
+        rf.reason = OFPRFR_GROUP_MOD;
+        rf.group_mod = &ogm->gm;
+        rf.new_buckets = new_group ? &new_group->buckets : NULL;
+        rf.group_existed = group_collection_n(&ogm->old_groups) > 0;
         connmgr_send_requestforward(ofproto->connmgr, req->ofconn, &rf);
     }
 }