LWT toplevel example: do not generate a source map

Outputting this source map would fail on 32-bit architectures since
its size, once Base64-encoded, would be over the string length limit
of 16 MiB.

Author: vouillon

toplevel/examples/lwt_toplevel/dune

@@ -39,7 +39,10 @@
   (:standard -linkall))
  (modes byte js)
  (js_of_ocaml
-  (link_flags (:standard))
+  (link_flags
+   :standard
+   ; The source map would be too large on 32-bit architectures otherwise
+   --no-source-map)
   (build_runtime_flags
    (:standard
     --file