Include benchmarks/*.csv with cabal sdist

[RyanGlScott]

Currently, the benchmarks fail to run when obtained from Hackage:

Running 1 benchmarks...
Benchmark benchmarks: RUNNING...
benchmarks: benchmarks/presidents.csv: openBinaryFile: does not exist (No such file or directory)
Benchmark benchmarks: ERROR

since cabal sdist doesn't grab the .csv files in the benchmarks directory. Adding them to extra-source-files does the trick.

See also iu-parfunc/sc-haskell#7

[hvr]

thanks!

[bergmark]

Can we get this released, please? Benchmarks are disabled in stackage because of this.