fix(build): add missing & required src/Import.resi

Fixes ocaml#556

Author: cdaringe

src/Import.resi

@@ -0,0 +1,20 @@
+type void
+module Jsonable: {
+  module type S1 = {
+    type t<'a>
+    let toJson: t<'a> => Js.Json.t
+    let ofJson: Js.Json.t => option<t<'a>>
+  }
+
+  module type S = {
+    type t
+    include S1 with type t<'a> := t
+  }
+
+  module Unsafe: {
+    // external assumeJsonable: 'a => Js.Json.t = "%identity"
+    let toJson: 'a => Js.Json.t
+    // external ofJson: Js.Json.t => 'a = "%identity"
+    let ofJson: Js.Json.t => option<'a>
+  }
+}