Restrict libudev

Upstream fix: Armael/ocaml-libudev#1

Author: emillon

packages/libudev/libudev.0.2.1/opam

@@ -11,7 +11,7 @@ depends: [
   "ocamlbuild" {build}
   "topkg" {build & >= "0.9.0"}
   "conf-libudev"
-  "ctypes" {>= "0.4.1"}
+  "ctypes" {>= "0.4.1" & < "0.21.0"}
   "ctypes-foreign"
   "stdint"
 ]