WaveformMark: fix incorrect destructor

thanks @fixxiefixx for the fixx

Signed-off-by: Owen Williams <owilliams@mixxx.org>

Author: ywwg

src/waveform/renderers/waveformmark.h

@@ -19,6 +19,7 @@ class WaveformMark {
   public:
     class Graphics {
       public:
+        virtual ~Graphics() = default;
         // To indicate that the image for the mark needs to be regenerated,
         // when the text, color, breadth or level are changed.
         bool m_obsolete{};