synthesize.c: Fix -Wmissing-prototypes warnings.

src/libespeak-ng/setlengths.c

 #include "synthesize.h"
 #include "translate.h"
 
-extern void DoSonicSpeed(int value);
 extern int saved_parameters[];
 
 // convert from words-per-minute to internal speed factor

src/libespeak-ng/synthdata.c

 int vowel_transition0;
 int vowel_transition1;
 
-int FormantTransition2(frameref_t *seq, int *n_frames, unsigned int data1, unsigned int data2, PHONEME_TAB *other_ph, int which);
-
 static espeak_ng_STATUS ReadPhFile(void **ptr, const char *fname, int *size, espeak_ng_ERROR_CONTEXT *context)
 {
 	if (!ptr) return EINVAL;

src/libespeak-ng/synthesize.h

 void SetEmbedded(int control, int value);
 void SelectPhonemeTable(int number);
 int  SelectPhonemeTableName(const char *name);
+int FormantTransition2(frameref_t *seq, int *n_frames, unsigned int data1, unsigned int data2, PHONEME_TAB *other_ph, int which);
 
 void Write4Bytes(FILE *f, int value);
 int Read4Bytes(FILE *f);
 int Reverse4Bytes(int word);
 
+#if HAVE_SONIC_H
+void DoSonicSpeed(int value);
+#endif
+
 #define ENV_LEN  128    // length of pitch envelopes
 #define PITCHfall   0  // standard pitch envelopes
 #define PITCHrise   2