Fix building stdthread.cpp for GCC 9.

- TStdThread destructor doesn't need to be virtual as the class is not used polymorphically
- Destructor of base class will be called automatically after derived one

src/gdlib/gmsstrm.cpp

@@ -117,7 +117,7 @@ void CompressTextFile( const std::string &fn, const std::string &fo, const std::
       NrRead = Fin.Read( Buffer.data(), (int) Buffer.size() );
       if( !NrRead ) break;
       Fout.Write( Buffer.data(), NrRead );
-   } while( NrRead >= static_cast<int>( Buffer.size() ) );
+   } while( NrRead >= static_cast<decltype(NrRead)>( Buffer.size() ) );
 }
 
 void UnCompressTextFile( const std::string &fn, const std::string &fo, const std::string &PassWord, int &ErrNr, std::string &ErrMsg )

src/rtl/stdthread.cpp

@@ -118,7 +118,6 @@ TStdThread::~TStdThread()
    {
       if( joinable() )
          detach();
-      std::thread::~thread();
    }
    catch( const std::exception & )
    {

src/rtl/stdthread.h

@@ -56,7 +56,7 @@ class TStdThread : public std::thread
 {
 public:
    explicit TStdThread(const std::function<void()> &execute);
-   virtual ~TStdThread();
+   ~TStdThread();
    [[nodiscard]] uint64_t getThreadHash() const;
 };