api: allow deprecated use of 'cause'

... because we don't want to bump our MSRV just to fix this.

src/error.rs

@@ -313,6 +313,7 @@ impl<W: ::std::any::Any> StdError for IntoInnerError<W> {
  self.err.description()
  }
 
+ #[allow(deprecated)]
  fn cause(&self) -> Option<&StdError> {
  self.err.cause()
  }