usage.c: remove reference to busybox.h
*: s/include "busybox.h"/include "libbb.h"

diff --git a/coreutils/df.c b/coreutils/df.c
index 484cabb..d455d27 100644
--- a/coreutils/df.c
+++ b/coreutils/df.c
@@ -24,7 +24,7 @@
 #include <unistd.h>
 #include <mntent.h>
 #include <sys/vfs.h>
-#include "busybox.h"
+#include "libbb.h"
 
 #ifndef CONFIG_FEATURE_HUMAN_READABLE
 static long kscale(long b, long bs)