klatt.c: Fix -Wmissing-prototypes warnings.

src/libespeak-ng/klatt.c

 static double klattp1[N_KLATTP];
 static double klattp_inc[N_KLATTP];
 
-int Wavegen_Klatt(int resume)
+static int Wavegen_Klatt(int resume)
 {
 	int pk;
 	int x;
 	return 0;
 }
 
-void SetSynth_Klatt(int length, frame_t *fr1, frame_t *fr2, voice_t *v, int control)
+static void SetSynth_Klatt(int length, frame_t *fr1, frame_t *fr2, voice_t *v, int control)
 {
 	int ix;
 	DOUBLEX next;

src/libespeak-ng/synthesize.h

 
 void InitBreath(void);
 
-void KlattInit();
+void KlattInit(void);
 void KlattReset(int control);
 int Wavegen_Klatt2(int length, int resume, frame_t *fr1, frame_t *fr2);