dispatch: add function to notify playback stopped

for the PC Speaker real driver

src/engine/platform/pcspkr.h

@@ -82,6 +82,7 @@ class DivPlatformPCSpeaker: public DivDispatch {
     bool keyOffAffectsArp(int ch);
     void setFlags(unsigned int flags);
     void notifyInsDeletion(void* ins);
+    void notifyPlaybackStop();
     void poke(unsigned int addr, unsigned short val);
     void poke(std::vector<DivRegWrite>& wlist);
     const char** getRegisterSheet();