Windows: Fix nullptr dereference when reopening plugin windows

On Windows, when opening a plugin editor, destroying the plugin
instance, and then creating a new instance and opening its editor, the
plugin would crash because the VBlankDispatcher singleton could not be
recreated.

modules/juce_gui_basics/native/juce_win32_Windowing.cpp

@@ -1573,7 +1573,7 @@ public:
                        threads.end());
     }
 
-    JUCE_DECLARE_SINGLETON_SINGLETHREADED (VBlankDispatcher, true)
+    JUCE_DECLARE_SINGLETON_SINGLETHREADED (VBlankDispatcher, false)
 
 private:
     //==============================================================================