[OCaml] Re-enable the using_protected unit test

Use the [member-variable] syntax in using_protected_runme.ml as
required by the OCaml module's documentation and implementation.

Fix the return type for vec_write() in example.h in the OCaml stl
example.

Examples/test-suite/ocaml/Makefile.in

@@ -39,7 +39,6 @@ nested_directors \
 preproc_constants \
 smart_pointer_inherit \
 typedef_mptr \
-using_protected \
 
 FAILING_C_TESTS = \
 enums \

Examples/test-suite/ocaml/using_protected_runme.ml

@@ -2,7 +2,7 @@ open Swig
 open Using_protected
 
 let f = new_FooBar C_void
-let _ = (invoke f) "x" (C_int 3)
+let _ = (invoke f) "[x]" (C_int 3)
 
 let _ = if (invoke f) "blah" (C_int 4) <> (C_int 4) then
     raise (Failure "blah(int)")