gdkwayland: Fix new clippy warning

```
 error: this expression creates a reference which is immediately dereferenced by the compiler
  --> gdkwayland/src/wayland_window.rs:48:34
   |
48 |             (*callback)(&window, &handle.as_str());
   |                                  ^^^^^^^^^^^^^^^^ help: change this to: `handle.as_str()`
   |
   = note: `-D clippy::needless-borrow` implied by `-D warnings`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow
```

Author: sdroege

gdkwayland/src/wayland_window.rs

@@ -49,7 +49,7 @@ impl WaylandWindow {
             let window = from_glib_borrow(window);
             let handle: Borrowed<glib::GString> = from_glib_borrow(handle);
             let callback: &P = &*(user_data as *mut _);
-            (*callback)(&window, &handle.as_str());
+            (*callback)(&window, handle.as_str());
         }
         unsafe extern "C" fn destroy_notify<P: Fn(&WaylandWindow, &str) + 'static>(
             data: glib::ffi::gpointer,