made SML/NJ happy;

--- a/src/Pure/library.ML	Thu Oct 22 17:09:29 2009 +0200
+++ b/src/Pure/library.ML	Thu Oct 22 17:54:47 2009 +0200
@@ -466,7 +466,7 @@
 
 fun map_range f i =
   let
-    fun mapp k =
+    fun mapp (k: int) =
       if k < i then f k :: mapp (k + 1) else [];
   in mapp 0 end;