[Encode] Move task manager m_closeMtx from public to protect

Author: Christa03

_studio/mfx_lib/encode_hw/shared/ehw_task_manager.h

@@ -56,9 +56,12 @@ namespace MfxEncodeHW
     private:
         std::map<mfxU32, std::deque<CachedBitstream>> m_cachedBitstream  = {};
         std::map<mfxU32, bool>                        m_outputReady      = {};
-        std::mutex                                    m_mtx, m_closeMtx;
+        std::mutex                                    m_mtx;
         std::condition_variable                       m_cv;
 
+    protected:
+        std::mutex                                    m_closeMtx;
+
     public:
 
         class TMRefWrapper