Add @iamphill to CODEOWNERS file

What does this MR do?

We've reinstated @iamphill as maintainer. So this MR adds him to .gitlab/CODEOWNERS file.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content