make SML/NJ happy;

--- a/src/HOL/Library/rewrite.ML	Thu Apr 16 20:54:01 2015 +0200
+++ b/src/HOL/Library/rewrite.ML	Thu Apr 16 23:01:33 2015 +0200
@@ -130,8 +130,8 @@
 
 in
 
-val ft_arg = ft_arg_gen arg_rewr_cconv
-val ft_imp = ft_arg_gen imp_rewr_cconv
+fun ft_arg ctxt = ft_arg_gen arg_rewr_cconv ctxt
+fun ft_imp ctxt = ft_arg_gen imp_rewr_cconv ctxt
 
 end