Bump copyright dates.

lib/zip_file_get_offset.c

diff --git a/lib/zip_file_get_offset.c b/lib/zip_file_get_offset.c
index 877c179..858a78a 100644
--- a/lib/zip_file_get_offset.c
+++ b/lib/zip_file_get_offset.c
@@ -1,6 +1,6 @@
 /*
   zip_file_get_offset.c -- get offset of file data in archive.
-  Copyright (C) 1999-2017 Dieter Baron and Thomas Klausner
+  Copyright (C) 1999-2018 Dieter Baron and Thomas Klausner
 
   This file is part of libzip, a library to manipulate ZIP archives.
   The authors can be contacted at <libzip@nih.at>