maintainability: remove unused externs

src/libespeak-ng/spect.c

 #include "spect.h"
 #include "ieee80.h"
 
-extern char voice_name[];
-
 static int frame_width;
 
 static int default_freq[N_PEAKS] =

src/libespeak-ng/synth_mbrola.c

 
 #ifdef INCLUDE_MBROLA
 
-extern unsigned char *outbuf;
-
 #if defined(_WIN32) || defined(_WIN64)
 #include <windows.h>
 #endif

src/libespeak-ng/voice.h

 
 } voice_t;
 
-// percentages shown to user, ix=N_PEAKS means ALL peaks
-extern unsigned short voice_pcnt[N_PEAKS+1][3];
-
 extern espeak_VOICE current_voice_selected;
 
 extern voice_t *voice;