BufWriter: actually export WriterPanicked error

I didn't notice the submodule, which means I failed to re-export this
to make it actually-public.
Reported-by: NAndrew Gallant <jamslam@gmail.com>
Signed-off-by: NIan Jackson <ijackson@chiark.greenend.org.uk>

library/std/src/io/buffered/mod.rs

@@ -14,6 +14,8 @@
 
 pub use bufreader::BufReader;
 pub use bufwriter::BufWriter;
+#[unstable(feature = "bufwriter_into_raw_parts", issue = "80690")]
+pub use bufwriter::WriterPanicked;
 pub use linewriter::LineWriter;
 use linewritershim::LineWriterShim;
 

library/std/src/io/mod.rs

@@ -264,6 +264,8 @@
 
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::buffered::IntoInnerError;
+#[unstable(feature = "bufwriter_into_raw_parts", issue = "80690")]
+pub use self::buffered::WriterPanicked;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::buffered::{BufReader, BufWriter, LineWriter};
 #[stable(feature = "rust1", since = "1.0.0")]