Ignore generated cabal.out and cabal.out''

[philderbeast]

I found an additional generated file that should be ignored.

Template Β: This PR does not modify cabal behaviour (documentation, tests, refactoring, etc.)

Include the following checklist in your PR:

• Patches conform to the coding conventions.

Without this change, after running the test locally we have:

$ 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)
	cabal-testsuite/PackageTests/NewUpdate/RejectFutureIndexStates/cabal.out''

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

[philderbeast]

Label merge+no rebase is necessary when the pull request is from an organisation.