Restrict zstd

Upstream fix: ygrek/ocaml-zstd#7

Author: emillon

packages/zstd/zstd.0.3/opam

@@ -17,7 +17,7 @@ build: [
 depends: [
   "ocaml"
   "dune" {>= "2.0"}
-  "ctypes"
+  "ctypes" {< "0.21.0"}
   "integers"
   ("extlib" {with-test} | "extlib-compat" {with-test})
   "base-unix" {with-test}