speech.c: Fix -Wmissing-prototypes warnings.

src/libespeak-ng/speech.c

 	return berr;
 }
 
-extern void FreePhData(void);
-
 ESPEAK_NG_API espeak_ng_STATUS espeak_ng_Terminate(void)
 {
 #ifdef USE_ASYNC

src/libespeak-ng/synthesize.h

 // from synthdata file
 unsigned int LookupSound(PHONEME_TAB *ph1, PHONEME_TAB *ph2, int which, int *match_level, int control);
 frameref_t *LookupSpect(PHONEME_TAB *this_ph, int which, FMT_PARAMS *fmt_params,  int *n_frames, PHONEME_LIST *plist);
+void FreePhData(void);
 
 unsigned char *LookupEnvelope(int ix);
 espeak_ng_STATUS LoadPhData(int *srate, espeak_ng_ERROR_CONTEXT *context);