fix for OCaml 5.1: use a generative functor for CCBitfield

src/data/CCBitField.ml

@@ -49,7 +49,7 @@ let get_then_incr n =
   incr n;
   x
 
-module Make (X : sig end) : S = struct
+module Make () : S = struct
   type t = int
 
   let empty = 0

src/data/CCBitField.mli

@@ -59,8 +59,8 @@ module type S = sig
   (** Current width of the bitfield. *)
 end
 
+module Make () : S
 (** Create a new bitfield type *)
-module Make : functor (_ : sig end) -> S
 
 (**/**)