code cleanup: remove unnecessary references to out_start.

Unsigned char *out_start is a global only used in speech.c.

src/libespeak-ng/klatt.c

 #include "klatt.h"
 
 extern unsigned char *out_ptr;
-extern unsigned char *out_start;
 extern unsigned char *out_end;
 static int nsamples;
 static int sample_count;

src/libespeak-ng/speech.c

 
 unsigned char *outbuf = NULL;
 int outbuf_size = 0;
+unsigned char *out_start;
 
 espeak_EVENT *event_list = NULL;
 int event_list_ix = 0;

src/libespeak-ng/synthesize.h

 extern SPEED_FACTORS speed;
 
 extern unsigned char *out_ptr;
-extern unsigned char *out_start;
 extern unsigned char *out_end;
 extern espeak_EVENT *event_list;
 extern t_espeak_callback *synth_callback;

src/libespeak-ng/wavegen.c

 static double two_pi_t;
 
 unsigned char *out_ptr;
-unsigned char *out_start;
 unsigned char *out_end;
 
 espeak_ng_OUTPUT_HOOKS* output_hooks = NULL;