Fix missing Windows change in ocaml#423

Author: dra27

tools/Makefile.nt

@@ -15,7 +15,7 @@ include Makefile.shared
 # To make custom toplevels
 
 OCAMLMKTOP=ocamlmktop.cmo
-OCAMLMKTOP_IMPORTS=misc.cmo identifiable.cmo numbers.cmo config.cmo clflags.cmo ccomp.cmo
+OCAMLMKTOP_IMPORTS=misc.cmo identifiable.cmo numbers.cmo config.cmo arg_helper.cmo clflags.cmo ccomp.cmo
 
 ocamlmktop: $(OCAMLMKTOP)
 	$(CAMLC) $(LINKFLAGS) -o ocamlmktop $(OCAMLMKTOP_IMPORTS) $(OCAMLMKTOP)