add 'ghc' as a --distro-pkg

Some hackage packages like 'hdevtools' specify 'ghc' as a hackage
package dependency. While we could just use a PKGBUILD or CABAL patch
for cblrepo, doing this is easy and doesn't really hurt how cblrepo
works, so it makes sense to do this.

Suggested-by: Eric Mrak <mail@ericmrak.info>