Include secondary LICENSE file in source dist

haddock-library/haddock-library.cabal

@@ -8,7 +8,8 @@ description:          Haddock is a documentation-generation tool for Haskell
                       project if you can't release often. For interacting with Haddock
                       itself, see the ‘haddock’ package.
 license:              BSD3
-license-file:         LICENSE
+license-files:        LICENSE
+                      vendor/attoparsec-0.13.1.0/LICENSE
 maintainer:           Alex Biehl <alexbiehl@gmail.com>, Simon Hengel <sol@typeful.net>, Mateusz Kowalczyk <fuuzetsu@fuuzetsu.co.uk>
 homepage:             http://www.haskell.org/haddock/
 bug-reports:          https://github.com/haskell/haddock/issues