std: Fix missing stability on prelude

The module itself is stable (the name)

src/libstd/prelude/mod.rs

@@ -35,5 +35,7 @@
 //! pervasive that it would be obnoxious to import for every use, particularly
 //! those that define methods on primitive types.
 
+#![stable]
+
 #[stable]
 pub mod v1;