oh whoops it does need to be cloned in that case

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tower/src/util/map_err.rs

@@ -40,7 +40,7 @@ where
 
     #[inline]
     fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
-        self.inner.poll_ready(cx).map_err(self.f)
+        self.inner.poll_ready(cx).map_err(self.f.clone())
     }
 
     #[inline]