app/internal/window: don't use module imports in framework header

Using the module import syntax "@import UIKit" results in a complaint
from the compiler:

> warning: use of '@import' in framework header is discouraged,
> including this header requires -fmodules [-Watimport-in-framework-header]

Use the regular #include syntax instead.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/window/framework_ios.h

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Unlicense OR MIT
 
-@import UIKit;
+#include <UIKit/UIKit.h>
 
 @interface GioAppDelegate : UIResponder <UIApplicationDelegate>
 @property (strong, nonatomic) UIWindow *window;