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]

.gitignore

@@ -125,6 +125,7 @@ _darcs/
 /docs/users_guide/utils.pyc
 /driver/ghci/ghc-pkg-inplace
 /driver/ghci/ghci-inplace
+/driver/ghci/ghci-wrapper.cabal
 /driver/ghci/ghci.res
 /driver/ghci/cwrapper.c
 /driver/ghci/cwrapper.h

driver/ghci/ghci-wrapper.cabal.in

+-- WARNING: ghci-wrapper.cabal is automatically generated from ghci-wrapper.cabal.in by
+-- ./configure.  Make sure you are editing ghci-wrapper.cabal.in, not ghci-wrapper.cabal.
+
 Name: ghci-wrapper
 Version: @ProjectVersion@
 Copyright: XXX
@@ -23,4 +26,4 @@ Executable ghci
     C-Sources:
        ghci.c
        -- the following get copied from ../utils by hadrian
-       getLocation.c isMinTTY.c cwrapper.c
\ No newline at end of file
+       getLocation.c isMinTTY.c cwrapper.c