Windows: Correctly dispose of hook

This fixes an issue where REAPER was unable to completely unload VST3
plugins.

modules/juce_gui_basics/native/juce_win32_Windowing.cpp

@@ -758,6 +758,11 @@ public:
        #endif
     }
 
+    ~NativeDarkModeChangeDetectorImpl()
+    {
+        UnhookWindowsHookEx (hook);
+    }
+
     bool isDarkModeEnabled() const noexcept  { return darkModeEnabled; }
 
 private: