Add *.xip to .gitignore #24598

We extract macOS SDK from XIP archives such as Xcode_12.2.xip. Let's add these into gitignore since we never want to commit these to the repository.

<!-- *** Please remove the following help text before submitting: *** Pull requests without a rationale and clear improvement may be closed immediately. GUI-related pull requests should be opened against https://github.com/bitcoin-core/gui first. See CONTRIBUTING.md -->

<!-- Please provide clear motivation for your patch and explain how it improves Bitcoin Core user experience or Bitcoin Core developer experience significantly: * Any test improvements or new tests that improve coverage are always welcome. * All other changes should have accompanying unit tests (see `src/test/`) or functional tests (see `test/`). Contributors should note which tests cover modified code. If no tests exist for a region of modified code, new tests should accompany the change. * Bug fixes are most welcome when they come with steps to reproduce or an explanation of the potential issue as well as reasoning for the way the bug was fixed. * Features are welcome, but might be rejected due to design or scope issues. If a feature is based on a lot of dependencies, contributors should first consider building the system outside of Bitcoin Core, if possible. * Refactoring changes are only accepted if they are required for a feature or bug fix or otherwise improve developer experience significantly. For example, most "code style" refactoring changes require a thorough explanation why they are useful, what downsides they have and why they *significantly* improve developer experience or avoid serious programming bugs. Note that code style is often a subjective matter. Unless they are explicitly mentioned to be preferred in the [developer notes](/doc/developer-notes.md), stylistic code changes are usually rejected. -->

<!-- Bitcoin Core has a thorough review process and even the most trivial change needs to pass a lot of eyes and requires non-zero or even substantial time effort to review. There is a huge lack of active reviewers on the project, so patches often sit for a long time. -->

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--174a7506f384e20aa4161008e828411d-->

Conflicts

Reviewers, this pull request conflicts with the following ones:

• #24031 (build: don't compress macOS DMG by fanquake)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Does https://github.com/bitcoin/bitcoin/blob/master/contrib/macdeploy/README.md#sdk-extraction assume that Xcode_12.2.xip resides in the bitcoin source tree?

Does https://github.com/bitcoin/bitcoin/blob/master/contrib/macdeploy/README.md#sdk-extraction assume that Xcode_12.2.xip resides in the bitcoin source tree?

No, it does not assume or imply it. Still, I think the change is worth getting in.

ACK 7215e3c00e227f4e703ca0aec8b048dd78c8eb3b.

~0 on this. In general we only add artifacts that our build process generates to the .gitignore. I know there are some historical exceptions, but we have explicitly rejected PRs that add say, meta files for some IDE or editor (the place to do this would be your local .gitignore). That said this is vaguely related so I'm not sure where this falls.

Ok. Let's just leave this for now. There's not really a reason to copy the .xip into the source tree either. In any case, hopefully it would be impossible for a multiple GB file to get added to a commit without anyone noticing.