fix objc macro return type inference failure

Author: antonok-edm

src/backend/vulkan/src/window.rs

@@ -274,14 +274,14 @@ impl Instance {
 
             if !use_current {
                 let layer: *mut Object = msg_send![class, new];
-                msg_send![view, setLayer: layer];
+                let () = msg_send![view, setLayer: layer];
                 let bounds: CGRect = msg_send![view, bounds];
-                msg_send![layer, setBounds: bounds];
+                let () = msg_send![layer, setBounds: bounds];
 
                 let window: *mut Object = msg_send![view, window];
                 if !window.is_null() {
                     let scale_factor: CGFloat = msg_send![window, backingScaleFactor];
-                    msg_send![layer, setContentsScale: scale_factor];
+                    let () = msg_send![layer, setContentsScale: scale_factor];
                 }
             }
         }