Fix implicit declaration of isatty(3) by including unistd.h

diff -upr vorbisgain-0.37.orig/misc.c vorbisgain-0.37/misc.c
--- vorbisgain-0.37.orig/misc.c	2023-04-23 10:30:54.226230204 +0200
+++ vorbisgain-0.37/misc.c	2023-04-23 10:31:16.529594158 +0200
@@ -14,6 +14,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 #include <vorbis/codec.h>
 #include "i18n.h"
 #include "misc.h"