Move polint from synthesize.h to spect.h.

src/libespeak-ng/spect.h

 {
 #endif
 
+float polint(float xa[], float ya[], int n, float x);
+
 #define FRAME_WIDTH  1000  // max width for 8000kHz frame
 #define MAX_DISPLAY_FREQ 9500
 #define FRAME_HEIGHT 240

src/libespeak-ng/synthesize.h

 int  WcmdqUsed(void);
 void WcmdqInc(void);
 void WavegenInit(int rate, int wavemult_fact);
-float polint(float xa[], float ya[], int n, float x);
 int WavegenFill(void);
 void MarkerEvent(int type, unsigned int char_position, int value, int value2, unsigned char *out_ptr);
 int GetAmplitude(void);