ocamlPackages.ocamlnet: fix build with OCaml 4.06

pkgs/development/ocaml-modules/ocamlnet/default.nix

@@ -1,5 +1,6 @@
 { stdenv, fetchurl, pkgconfig, ncurses, ocaml, findlib, ocaml_pcre, camlzip
-, gnutls, nettle }:
+, gnutls, nettle, fetchpatch
+}:
 
 let version = "4.1.5"; in
 
@@ -11,6 +12,12 @@ stdenv.mkDerivation {
     sha256 = "1ppcd2zjhj6s3ib9q8dngnr53qlmkhvv7a8hzp88r79k6jygn4cm";
   };
 
+  patches = [ (fetchpatch {
+      url = "https://raw.githubusercontent.com/ocaml/opam-repository/master/packages/ocamlnet/ocamlnet.4.1.5/files/netgzip.patch";
+      sha256 = "1say7zzgk24qcy9m91gcfgvz4fv7nksx4j5qnbxyq8wqw0g88ba0";
+    })
+  ];
+
   nativeBuildInputs = [ pkgconfig ];
   buildInputs = [ ncurses ocaml findlib ocaml_pcre camlzip gnutls nettle ];