Fix forgotten license comments

diff --git a/util-linux/rtcwake.c b/util-linux/rtcwake.c
index a9766ca..6c399c7 100644
--- a/util-linux/rtcwake.c
+++ b/util-linux/rtcwake.c
@@ -3,6 +3,8 @@
  *
  * This version was taken from util-linux and scrubbed down for busybox.
  *
+ * Licensed under GPLv2, see file LICENSE in this tarball for details.
+ *
  * This uses cross-platform Linux interfaces to enter a system sleep state,
  * and leave it no later than a specified time.  It uses any RTC framework
  * driver that supports standard driver model wakeup flags.