Add a virtual destructor to the TtsEngine class.
diff --git a/include/tts/TtsEngine.h b/include/tts/TtsEngine.h
index 21cb73b..ed084ca 100644
--- a/include/tts/TtsEngine.h
+++ b/include/tts/TtsEngine.h
@@ -80,6 +80,8 @@
 class TtsEngine
 {
 public:
+    virtual ~TtsEngine() {}
+
     // Initialize the TTS engine and returns whether initialization succeeded.
     // @param synthDoneCBPtr synthesis callback function pointer
     // @return TTS_SUCCESS, or TTS_FAILURE