Fix forgotten license comments

diff --git a/libbb/update_passwd.c b/libbb/update_passwd.c
index 88bc28c..565dd37 100644
--- a/libbb/update_passwd.c
+++ b/libbb/update_passwd.c
@@ -7,6 +7,8 @@
  * /etc/shadow) for a given user and password.
  *
  * Moved from loginutils/passwd.c by Alexander Shishkin <virtuoso@slind.org>
+ *
+ * Licensed under GPLv2, see file LICENSE in this tarball for details.
  */
 
 #include "libbb.h"