qualify identifiers passed through largefile_fn

This makes largefile_fn produce qualified identifiers (e.g. libc::open
instead of open).

Author: inglorion

src/macros.rs

@@ -367,7 +367,7 @@ cfg_if! {
         /// identifier of the corresponding I/O function with large file
         /// support.
         macro_rules! largefile_fn {
-            [$id:ident] => ($id);
+            [$id:ident] => (libc::$id);
         }
     }
 }