[image_picker] Add missing entitlements to macOS example

[stuartmorgan]

When endorsing macOS and adding the example app, I forgot to add the required entitlement, so the example didn't work.

The actual fix is the entitlements file; the Runner.xc* changes are auto-generated by running flutter build macos; usually we do that before landing examples but clearly I didn't remember to actually run the example when endorsing in this case...

Fixes flutter/flutter#134817

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie or stuartmorgan on the #hackers channel in Chat (don't just cc them here, they won't see it! Use Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[stuartmorgan]

This currently can't be tested due to flutter/flutter#70234

Once that's implemented and we can do the UI testing backfill for this plugin, this will automatically be covered.

[stuartmorgan]

No changelog: This only affects running locally, and is doing what the README already says to do.

[stuartmorgan]

test-exempt: will be automatically covered once necessary test infrastructure lands

[tarrinneal]

TAMNTM

[stuartmorgan]

Re-running repo_checks doesn't seem to be picking up the override at the moment for some reason. I'm going to force-land, but I'll look into that later today or next week.