Fix for crash when exiting whilst NewGRF scan still in progress

See: https://github.com/OpenTTD/OpenTTD/issues/8760

src/progress.h

@@ -40,6 +40,7 @@ static inline bool UseThreadedModelProgress()
 bool IsFirstModalProgressLoop();
 void SetModalProgress(bool state);
 void SleepWhileModalProgress(int milliseconds);
+void WaitUntilModalProgressCompleted();
 
 extern std::mutex _modal_progress_work_mutex;
 extern std::mutex _modal_progress_paint_mutex;