std: Add missing stability for symlink functions

These functions were intended to be introduced as `#[stable]` as a stable API
was deprecated in favor of them, but they just erroneously forgot the stability
attributes.

Author: alexcrichton

src/libstd/sys/unix/ext.rs

@@ -249,6 +249,7 @@ pub mod fs {
     /// # Ok(())
     /// # }
     /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
     pub fn symlink<P: AsRef<Path>, Q: AsRef<Path>>(src: P, dst: Q) -> io::Result<()>
     {
         sys::fs2::symlink(src.as_ref(), dst.as_ref())

src/libstd/sys/windows/ext.rs

@@ -256,6 +256,7 @@ pub mod fs {
     /// # Ok(())
     /// # }
     /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
     pub fn symlink_file<P: AsRef<Path>, Q: AsRef<Path>>(src: P, dst: Q)
                                                         -> io::Result<()>
     {
@@ -278,6 +279,7 @@ pub mod fs {
     /// # Ok(())
     /// # }
     /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
     pub fn symlink_dir<P: AsRef<Path>, Q: AsRef<Path>> (src: P, dst: Q)
                                                         -> io::Result<()>
     {