Restrict memcpy

Upstream fix: yallop/ocaml-memcpy#11

Author: emillon

packages/memcpy/memcpy.0.2.1/opam

@@ -12,7 +12,7 @@ install: [[make "install"]]
 remove: [["ocamlfind" "remove" "memcpy"]]
 depends: [
   "ocaml" {>= "4.01.0"}
-  "ctypes" {>= "0.12.0"}
+  "ctypes" {>= "0.12.0" & < "0.21.0"}
   "ounit" {with-test}
   "ocamlfind" {build}
   "ocamlbuild" {build}