KFormat: Replace unicode literal with unicode codepoint to fix MSVC build

Summary:
MSVC is unable to parse UTF-8 sequences when the file has no BOM (byte
order mark), and fails with "error C2015: too many characters in constant".

Test Plan: run kformattest unit test

Reviewers: Frameworks, kossebau, bcooksley

Reviewed By: bcooksley

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14122