Core/Game: Move singleton instances into compilation units

* Fixes issues when building shared libraries
  (prevents gcc and clang from providing several instance)

(cherry picked from commit 329225b40d)

src/server/game/Maps/MapManager.cpp

@@ -62,6 +62,12 @@ void MapManager::InitializeVisibilityDistanceInfo()
         (*iter).second->InitVisibilityDistance();
 }
 
+MapManager* MapManager::instance()
+{
+    static MapManager instance;
+    return &instance;
+}
+
 Map* MapManager::CreateBaseMap(uint32 id)
 {
     Map* map = FindBaseMap(id);