make it work when no node-modules are installed

Author: jfrolich

scripts/buildRuntimeRewatch.sh

@@ -12,4 +12,5 @@ cp runtime/lib/js/*.js lib/js
 cp runtime/lib/bs/*.@(cmi|cmj|cmt|cmti) lib/ocaml/
 cp runtime/*.@(res|resi) lib/ocaml/
 # copy the stdlib build artifacts to the testrepo
+mkdir -p rewatch/testrepo/node_modules/rescript/lib/ocaml
 cp -rf lib/ocaml rewatch/testrepo/node_modules/rescript/lib/ocaml