Add cabal to build-tool-depends

[nponeccop]

@DanielG wrote in haskell/haskell-ide-engine#372 :

The dependency on the cabal executable is declared in Setup.hs but that only applies to Cabal < 2 so I guess we should probably add cabal to build-tool-depends actually.

[DanielG]

We have build-tool-depends: cabal-install:cabal in the (hopefully) appropriate places now: https://github.com/DanielG/cabal-helper/blob/master/cabal-helper.cabal#L144. This got released in cabal-helper-0.8.0.0.