debian: provide dates in copyright.

Fixes copyright-without-copyright-notice lintian warning.
diff --git a/debian/copyright b/debian/copyright
index 804bf7c..83258f0 100644
--- a/debian/copyright
+++ b/debian/copyright
@@ -12,7 +12,8 @@
 
 Copyright:
 
- Copyright (C) Igor Sysoev
+Copyright (C) 2002-2016 Igor Sysoev
+Copyright (C) 2011-2016 Nginx, Inc.
 
 License: