Platform: more unused variable warning fixes on macOS.

Followup to cb568c09e5, where cleaning up
one unused variable led to another variable now being unused. Hopefully
this time it's all.

src/Magnum/Platform/GlfwApplication.cpp

@@ -184,6 +184,8 @@ Vector2 GlfwApplication::dpiScalingInternal(const Implementation::GlfwDpiScaling
     } else {
         #ifndef CORRADE_TARGET_APPLE
         dpiScalingPolicy = configurationDpiScalingPolicy;
+        #else
+        static_cast<void>(configurationDpiScalingPolicy);
         #endif
     }
 

src/Magnum/Platform/Sdl2Application.cpp

@@ -318,6 +318,8 @@ Vector2 Sdl2Application::dpiScalingInternal(const Implementation::Sdl2DpiScaling
     } else {
         #ifndef CORRADE_TARGET_APPLE
         dpiScalingPolicy = configurationDpiScalingPolicy;
+        #else
+        static_cast<void>(configurationDpiScalingPolicy);
         #endif
     }