voices.c: Fix -Wmissing-prototypes warnings.

src/libespeak-ng/speech.c

 }
 
 extern void FreePhData(void);
-extern void FreeVoiceList(void);
 
 ESPEAK_NG_API espeak_ng_STATUS espeak_ng_Terminate(void)
 {

src/libespeak-ng/voice.h

 void WavegenSetVoice(voice_t *v);
 void ReadTonePoints(char *string, int *tone_pts);
 void VoiceReset(int control);
+void FreeVoiceList(void);
 
 #ifdef __cplusplus
 }