Move outbuf_size to where it is actually used.

src/libespeak-ng/speech.c

 #include "event.h"
 
 unsigned char *outbuf = NULL;
+int outbuf_size = 0;
 
 espeak_EVENT *event_list = NULL;
 int event_list_ix = 0;

src/libespeak-ng/synthesize.h

 extern SPEED_FACTORS speed;
 
 extern long count_samples;
-extern int outbuf_size;
 extern unsigned char *out_ptr;
 extern unsigned char *out_start;
 extern unsigned char *out_end;

src/libespeak-ng/wavegen.c

 unsigned char *out_ptr;
 unsigned char *out_start;
 unsigned char *out_end;
-int outbuf_size = 0;
 
 // the queue of operations passed to wavegen from sythesize
 intptr_t wcmdq[N_WCMDQ][4];