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/Scripting/ScriptSystem.cpp

@@ -23,6 +23,12 @@
 
 ScriptPointVector const SystemMgr::_empty;
 
+SystemMgr* SystemMgr::instance()
+{
+    static SystemMgr instance;
+    return &instance;
+}
+
 void SystemMgr::LoadScriptWaypoints()
 {
     uint32 oldMSTime = getMSTime();