qapi-visit: Use common idiom in gen_visit_fields_decl()

We have several instances of methods that do an early exit if
output is not needed, then log that output is being generated,
and finally produce the output; see qapi-types.py:gen_object()
and qapi-visit.py:gen_visit_implicit_struct().  The odd man
out was gen_visit_fields_decl(); rearrange it to be more like
the others.  No semantic change or difference to generated code.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <1455778109-6278-12-git-send-email-eblake@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>

scripts/qapi-visit.py

@@ -35,15 +35,14 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_type)sobj, Error **
 
 
 def gen_visit_fields_decl(typ):
-    ret = ''
-    if typ.name not in struct_fields_seen:
-        ret += mcgen('''
+    if typ.name in struct_fields_seen:
+        return ''
+    struct_fields_seen.add(typ.name)
+    return mcgen('''
 
 static void visit_type_%(c_type)s_fields(Visitor *v, %(c_type)s *obj, Error **errp);
 ''',
-                     c_type=typ.c_name())
-        struct_fields_seen.add(typ.name)
-    return ret
+                 c_type=typ.c_name())
 
 
 def gen_visit_implicit_struct(typ):