build: remove (useless) reference to Stdlib in mli too!

lib/lv6MainArgs.mli

-(* Time-stamp: <modified the 05/07/2021 (at 10:29) by Erwan Jahier> *)
+(* Time-stamp: <modified the 13/07/2021 (at 11:44) by Erwan Jahier> *)
 
 type enum_mode = 
     AsInt  (* translate enums into int (for rif-friendlyness *)
@@ -38,7 +38,7 @@ type t = {
   mutable expand_arrays :  bool;
   mutable expand_io_type :  bool;
   mutable optim_ite : bool;
-  mutable oc :  Stdlib.out_channel;
+  mutable oc :  out_channel;
   mutable tlex :  bool;
   mutable exec :  bool;
   mutable gen_c :  bool;