headers: move WordToString() to synthesize.h

src/libespeak-ng/phoneme.h

 #define PH3(c1, c2, c3) (c3<<16)+(c2<<8)+c1
 #define PhonemeCode2(c1, c2) PhonemeCode((c2<<8)+c1)
 
-extern const char *WordToString(unsigned int word);
-
 extern PHONEME_TAB_LIST phoneme_tab_list[N_PHONEME_TABS];
 extern int phoneme_tab_number;
 

src/libespeak-ng/synthesize.h

 int DoSample3(PHONEME_DATA *phdata, int length_mod, int amp);
 int DoSpect2(PHONEME_TAB *this_ph, int which, FMT_PARAMS *fmt_params,  PHONEME_LIST *plist, int modulation);
 int PauseLength(int pause, int control);
+const char *WordToString(unsigned int word);
 
 #ifdef __cplusplus
 }