Prevent dangling pointers to meters...

... This stops a crash that might occur when the checkpoint thread causes
ProjectAudioManager::Stop() to execute but the project is in the process of
closing.

src/ProjectAudioIO.cpp

@@ -12,6 +12,7 @@ Paul Licameli split from AudacityProject.cpp
 
 #include "AudioIOBase.h"
 #include "Project.h"
+#include "widgets/MeterPanelBase.h"
 
 static const AudacityProject::AttachedObjects::RegisteredFactory sAudioIOKey{
   []( AudacityProject &parent ){