[generators] gen_periodic: avoid unused arg warning

don't create var j if there are no messages in a mode

sw/tools/generators/gen_periodic.ml

@@ -41,8 +41,6 @@ let output_modes = fun out_h process_name telem_type modes freq modules ->
   let min_period = 1./.float freq in
   let max_period = 65536. /. float freq in
 
-  lprintf out_h "uint8_t j;\n";
-
   (** For each mode in this process *)
   List.iter
     (fun mode ->
@@ -68,6 +66,10 @@ let output_modes = fun out_h process_name telem_type modes freq modules ->
         lprintf out_h "static %s %s = 0; %s++; if (%s>=%d) %s=0;\n" _type v v v m v;
       ) modulos;
 
+      (* create var to loop trough callbacks if needed *)
+      if (List.length messages > 0) then
+        lprintf out_h "uint8_t j;\n";
+
       (** For each message in this mode *)
       let messages = List.sort (fun (_,p) (_,p') -> compare p p') messages in
       let i = ref 0 in (** Basic balancing:1 message every 10Hz *)