sexp_decode: Apply suggestions from code review

Author: mseri

packages/sexp_decode/sexp_decode.0.2/opam

@@ -8,8 +8,9 @@ license: "LGPL-3.0-or-later"
 homepage: "https://gitlab.inria.fr/bmontagu/sexp_decode"
 bug-reports: "Benoît Montagu <[email protected]>"
 depends: [
+  "ocaml" {>= "4.08"}
   "csexp" {>= "1.5.1"}
-  "dune" {>= "2.9" & build >= "2.9.0"}
+  "dune" {>= "2.9"}
   "odoc" {with-doc}
 ]
 build: [