ocplib-json-typed: Makefile update for dune

We don't care as we're not using the Makefile directly but:
dune: --dev is no longer accepted as it is now the default.

Makefile

@@ -1,5 +1,5 @@
 all:
-	dune build @install @runtest --dev
+	dune build @install @runtest
 
 install:
 	dune install