Add `alex` as required `build-tool` in `fast-tags.cabal`
Add `alex` as required `build-tool` in `fast-tags.cabal`

The main reason that I'm adding this is that this benefits Nix.  The `cabal2nix`
tool builds a Nix expression from the corresponding `fast-tags.cabal` file of a
Haskell project, and it requires this `build-tool: alex` field to know to add
`alex` as a project dependency.

This may or may not also help plain `cabal`-only builds, but it can't hurt.

elaforge/fast-tags,elaforge/fast-tags