Add benchmark and test modules to extra-source-files.

`cabal sdist` doesn't include bench and test modules.

Fixes msgpack#20
Fixes msgpack#25

Author: iphydf

data-msgpack.cabal

@@ -18,6 +18,12 @@ description:
   since the original author is unreachable. This fork incorporates a number of
   bugfixes and is actively being developed.
 
+extra-source-files:
+  bench/Data/MessagePack/*.hs
+  bench/Data/*.hs
+  test/Data/MessagePack/*.hs
+  test/Data/*.hs
+
 source-repository head
   type:             git
   location:         https://github.com/TokTok/hs-msgpack.git