GameListCtrl: Put CACHE_REVISION in .cpp file

Reduces the number of files that have to be recompiled
when changing the cache revision.

Source/Core/DolphinWX/GameListCtrl.cpp

@@ -80,6 +80,8 @@ public:
   wxProgressDialog* dialog;
 };
 
+static constexpr u32 CACHE_REVISION = 2;  // Last changed in PR 5687
+
 static bool sorted = false;
 
 static int CompareGameListItems(const GameListItem* iso1, const GameListItem* iso2,

Source/Core/DolphinWX/GameListCtrl.h

@@ -125,7 +125,6 @@ private:
   } m_image_indexes;
 
   // Actual backing GameListItems are maintained in a background thread and cached to file
-  static constexpr u32 CACHE_REVISION = 2;  // Last changed in PR 5687
   std::list<std::shared_ptr<GameListItem>> m_cached_files;
   // Locks the list, not the contents
   std::mutex m_cache_mutex;