Add driver/ghci/ghci-wrapper.cabal to .gitignore

After commit 55ef3bdc, running `./configure`
now generates a `driver/ghci/ghci-wrapper.cabal` file from
`driver/ghci/ghci-wrapper.cabal.in`, which pollutes the `git` tree:

```
$ git status
On branch master
Your branch is up to date with 'origin/master'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        driver/ghci/ghci-wrapper.cabal

nothing added to commit but untracked files present (use "git add" to track)
```

Since `driver/ghci/ghci-wrapper.cabal` is autogenerated, the sensible thing to
do is to add it to `.gitignore`. While I was in town, I also added the standard
`*.in` file disclaimer to `driver/ghci/ghci-wrapper.cabal.in`.

[ci skip]