use "glibc errno" trick not only for ash, but for entire busybox

(add/remove: 1/1 grow/shrink: 37/37 up/down: 139/-228)        Total: -89 bytes

debianutils/mktemp.c

@@ -10,11 +10,6 @@
  */
 
 #include "libbb.h"
-#include <stdio.h>
-#include <errno.h>
-#include <string.h>
-#include <unistd.h>
-#include <stdlib.h>
 
 int mktemp_main(int argc, char **argv);
 int mktemp_main(int argc, char **argv)