Fix forgotten license comments

diff --git a/applets/usage.c b/applets/usage.c
index a35817f..1e038b3 100644
--- a/applets/usage.c
+++ b/applets/usage.c
@@ -1,4 +1,9 @@
 /* vi: set sw=4 ts=4: */
+/*
+ * Copyright (C) 2008 Denys Vlasenko.
+ *
+ * Licensed under GPLv2, see file LICENSE in this tarball for details.
+ */
 #include <unistd.h>
 
 /* Just #include "autoconf.h" doesn't work for builds in separate