usage.c: remove reference to busybox.h

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

miscutils/bbconfig.c

@@ -1,7 +1,7 @@
 /* vi: set sw=4 ts=4: */
 /* This file was released into the public domain by Paul Fox.
  */
-#include "busybox.h"
+#include "libbb.h"
 #include "bbconfigopts.h"
 
 int bbconfig_main(int argc, char **argv);