chore: fix SwiftPM warnings

[kkebo]

This pull request fixes the following warnings from SwiftPM.

$ swift build
...
warning: 'swift-format': found 1 file(s) which are unhandled; explicitly declare them as resources or exclude from the target
    /home/kebo/swift-format/Sources/swift-format/CMakeLists.txt
warning: 'swift-format': found 1 file(s) which are unhandled; explicitly declare them as resources or exclude from the target
    /home/kebo/swift-format/Sources/SwiftFormat/CMakeLists.txt
...

Those files have been added in #677.

I confirmed that this PR fixed warnings in Swift 5.10, 6.0, and the main snapshot.