Use filtered instead of filter in rustdoc. Fixes segfault.

...of course, it shouldn't be segfaulting, but this fixes the
build.

rs=breakage

Author: catamorphism

src/librustdoc/prune_private_pass.rs

@@ -49,7 +49,7 @@ fn fold_mod(
     let doc = fold::default_any_fold_mod(fold, doc);
 
     doc::ModDoc_({
-        items: doc.items.filter(|ItemTag| {
+        items: doc.items.filtered(|ItemTag| {
             is_visible(fold.ctxt, ItemTag.item())
         }),
         .. *doc