Add `hsc2hs` as a dependency

[vmchale]

hsc2hs should be listed in the build-tool-depends section of the .cabal file - otherwise builds can fail with

Failed to build basement-0.0.8.
Build log (
/home/vanessa/.cabal/logs/ghc-8.6.2/basement-0.0.8-778369c941d299194dfe929357824b62256ebc11fd6719f7e6a3a653a447aab4.log
):
Configuring library for basement-0.0.8..
Preprocessing library for basement-0.0.8..
cabal: The program 'hsc2hs' is required but it could not be found.

cabal: Failed to build basement-0.0.8 (which is required by exe:cpkg from
cpkg-0.1.0.0). See the build log above for details.