usage.c: remove reference to busybox.h

*: s/include "busybox.h"/include "libbb.h"

init/halt.c

@@ -7,7 +7,7 @@
  * Licensed under GPL version 2, see file LICENSE in this tarball for details.
  */
 
-#include "busybox.h"
+#include "libbb.h"
 #include <sys/reboot.h>
 
 int halt_main(int argc, char **argv);

init/init.c

@@ -9,7 +9,7 @@
  * Licensed under GPLv2 or later, see file LICENSE in this tarball for details.
  */
 
-#include "busybox.h"
+#include "libbb.h"
 #include <paths.h>
 #include <signal.h>
 #include <sys/ioctl.h>

init/mesg.c

@@ -7,7 +7,7 @@
  * Licensed under the GPL v2 or later, see the file LICENSE in this tarball.
  */
 
-#include "busybox.h"
+#include "libbb.h"
 
 #ifdef USE_TTY_GROUP
 #define S_IWGRP_OR_S_IWOTH	S_IWGRP