UI: Add missing function declaration

This broke the build, but due to an issue with CI this was not caught by
CI.

UI/platform.hpp

@@ -65,4 +65,5 @@ RunOnceMutex GetRunOnceMutex(bool &already_running);
 #ifdef __APPLE__
 void EnableOSXVSync(bool enable);
 void EnableOSXDockIcon(bool enable);
+void InstallNSApplicationSubclass();
 #endif