Fix 'elided lifetime' warning from nightly

Nightly 1.90. The fix passes on stable 1.88.

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

tokio-xmpp/src/stanzastream/worker.rs

@@ -334,7 +334,7 @@ impl WorkerStream {
         }
     }
 
-    fn close(&mut self) -> Close {
+    fn close(&mut self) -> Close<'_> {
         Close {
             stream: Pin::new(self),
         }