Fix the SPM issue with ObjC headers in WPKit

[kean]

I don't think SWIFT_PACKAGE macro works in Objective-C headers, and that what was causing the problem. That was the only header file using it. No header – no problem.

Note: to fully work, it will also need this https://github.com/wordpress-mobile/WordPress-iOS-Shared/pull/361 – WIP.

To test:

Regression Notes

1. Potential unintended areas of impact

2. What I did to test those areas of impact (or what existing automated tests I relied on)

3. What automated tests I added (or what prevented me from doing so)

PR submission checklist:

• [ ] I have completed the Regression Notes.
• [ ] I have considered adding unit tests for my changes.
• [ ] I have considered adding accessibility improvements for my changes.
• [ ] I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Testing checklist:

• [ ] WordPress.com sites and self-hosted Jetpack sites.
• [ ] Portrait and landscape orientations.
• [ ] Light and dark modes.
• [ ] Fonts: Larger, smaller and bold text.
• [ ] High contrast.
• [ ] VoiceOver.
• [ ] Languages with large words or with letters/accents not frequently used in English.
• [ ] Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• [ ] iPhone and iPad.
• [ ] Multi-tasking: Split view and Slide over. (iPad)

[dangermattic]

	1 Warning
:warning:	PR is not assigned to a milestone.

Generated by :no_entry_sign: Danger

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr23390-9ed3e33
	Version	25.1
	Bundle ID	org.wordpress.alpha
	Commit	9ed3e33ad9fc897ed2b47f8edbf6aae9b8922c46
	App Center Build	WPiOS - One-Offs #10231

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr23390-9ed3e33
	Version	25.1
	Bundle ID	com.jetpack.alpha
	Commit	9ed3e33ad9fc897ed2b47f8edbf6aae9b8922c46
	App Center Build	jetpack-installable-builds #9280

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.