speech.c: Fix -Wmissing-prototypes warnings.

src/include/espeak-ng/speak_lib.h

 #define ESPEAK_API
 #endif
 
-#define ESPEAK_API_REVISION  10
+#define ESPEAK_API_REVISION  12
 /*
 Revision 2
    Added parameter "options" to eSpeakInitialize()
 Revision 10 29.Aug.2014
   Changed phonememode parameter to espeak_TextToPhonemes() and espeak_SetPhonemeTrace
 
+Revision 11 (espeak-ng)
+  Made ESPEAK_API import/export symbols correctly on Windows.
+
+Revision 12 (espeak-ng)
+  Exposed espeak_SetPhonemeCallback. This is available in eSpeak, but was not exposed in this header.
+
 */
          /********************/
          /*  Initialization  */
              occurs.  The calling program can then play the sound at that point.
 */
 
+#ifdef __cplusplus
+extern "C"
+#endif
+ESPEAK_API void espeak_SetPhonemeCallback(int (*PhonemeCallback)(const char *));
+
 
          /********************/
          /*    Synthesis     */