gtk: Fix new clippy warning

```
 error: this expression creates a reference which is immediately dereferenced by the compiler
  --> gtk/src/stack_switcher.rs:52:15
   |
52 |             f(&StackSwitcher::from_glib_borrow(this).unsafe_cast_ref())
   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: change this to: `StackSwitcher::from_glib_borrow(this).unsafe_cast_ref()`
   |
   = 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

gtk/src/stack_switcher.rs

@@ -65,7 +65,7 @@ impl<O: IsA<StackSwitcher>> StackSwitcherExtManual for O {
             f: glib::ffi::gpointer,
         ) {
             let f: &F = &*(f as *const F);
-            f(&StackSwitcher::from_glib_borrow(this).unsafe_cast_ref())
+            f(StackSwitcher::from_glib_borrow(this).unsafe_cast_ref())
         }
         unsafe {
             let f: Box_<F> = Box_::new(f);