revert change that broke 4.03

Author: c-cube

src/threads/CCPool.mli

@@ -14,8 +14,10 @@ end
 
 exception Stopped
 
+[@@@ocaml.warning "-67"]
+
 (** {2 Create a new Pool} *)
-module Make (_ : PARAM) : sig
+module Make (P : PARAM) : sig
   val run : (unit -> _) -> unit
   (** [run f] schedules [f] for being executed in the thread pool. *)