sockopt: fix windows build

Author: avsm

lib_eio_windows/flow.ml

@@ -81,6 +81,12 @@ let of_fd fd = object (_ : <Eio_unix.Net.stream_socket; Eio.File.rw>)
   method probe : type a. a Eio.Generic.ty -> a option = function
     | Eio_unix.Resource.FD -> Some fd
     | _ -> None
+
+  method getsockopt : type a. a Eio.Net.Sockopt.t -> a = fun opt ->
+    Eio.Net.Sockopt.get fd opt
+
+  method setsockopt : type a. a Eio.Net.Sockopt.t -> a -> unit = fun opt v ->
+    Eio.Net.Sockopt.set fd opt v
 end
 
 let secure_random = object