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

coreutils/mv.c

@@ -15,10 +15,7 @@
 
 #include <sys/types.h>
 #include <sys/stat.h>
-#include <unistd.h>
 #include <dirent.h>
-#include <errno.h>
-#include <stdlib.h>
 #include <getopt.h> /* struct option */
 #include "libbb.h"
 #include "libcoreutils/coreutils.h"