Fix elided_named_lifetimes warning

Rustc nightly started warning e.g.:

```
  --> x11rb-async/src/connection.rs:70:30
   |
67 |     fn send_trait_request_with_reply<'this, 'req, 'future, R>(
   |                                      ----- lifetime `'this` declared here
...
70 |     ) -> Fut<'future, Cookie<'_, Self, R::Reply>, ConnectionError>
   |                              ^^ this elided lifetime gets resolved as `'this`
   |
   = note: `#[warn(elided_named_lifetimes)]` on by default
```

Signed-off-by: Uli Schlachter <psychon@znc.in>

x11rb-async/src/connection.rs

@@ -67,7 +67,7 @@ pub trait RequestConnection: Sync {
     fn send_trait_request_with_reply<'this, 'req, 'future, R>(
         &'this self,
         request: R,
-    ) -> Fut<'future, Cookie<'_, Self, R::Reply>, ConnectionError>
+    ) -> Fut<'future, Cookie<'this, Self, R::Reply>, ConnectionError>
     where
         'this: 'future,
         'req: 'future,
@@ -131,7 +131,7 @@ pub trait RequestConnection: Sync {
     fn send_trait_request_with_reply_with_fds<'this, 'req, 'future, R>(
         &'this self,
         request: R,
-    ) -> Fut<'future, CookieWithFds<'_, Self, R::Reply>, ConnectionError>
+    ) -> Fut<'future, CookieWithFds<'this, Self, R::Reply>, ConnectionError>
     where
         'this: 'future,
         'req: 'future,