remove unnecessary include of synthesize.h

src/libespeak-ng/synthdata.h

 #ifndef ESPEAK_NG_SYNTHDATA_H
 #define ESPEAK_NG_SYNTHDATA_H
 
+#include "synthesize.h"
 #include "translate.h"
 
 #ifdef __cplusplus
 {
 #endif
 
-// forward declare these to avoid including the whole synthesize.h
-struct PHONEME_LIST;
-struct PHONEME_DATA;
-struct WORD_PH_DATA;
-
 void InterpretPhoneme(Translator *tr, int control, PHONEME_LIST *plist, PHONEME_DATA *phdata, WORD_PH_DATA *worddata);
 void InterpretPhoneme2(int phcode, PHONEME_DATA *phdata);
 

src/libespeak-ng/translate.h

 #include <espeak-ng/espeak_ng.h>
 #include <espeak-ng/encoding.h>
 
-#include "synthesize.h"
-
 #ifdef __cplusplus
 extern "C"
 {