add virtual destructor for SkExample to fix build error

experimental/SkiaExamples/SkExample.h

Index: experimental/SkiaExamples/SkExample.h
diff --git a/experimental/SkiaExamples/SkExample.h b/experimental/SkiaExamples/SkExample.h
index bea3aec4fc10d2f8926a8e17b818d44557a5dc18..4b84723ac2435546abb260e74ca9d29b989379b8 100644
--- a/experimental/SkiaExamples/SkExample.h
+++ b/experimental/SkiaExamples/SkExample.h
@@ -23,6 +23,8 @@ class SkExample : public SkNoncopyable {
 public:
     SkExample(SkExampleWindow* window) : fWindow(window) {}
 
+    virtual ~SkExample() {}
+
     // Your class should override this method to do its thing.
     virtual void draw(SkCanvas* canvas) = 0;