Enable API breakage check when SwiftPM no longer diagnoses SPI changes as breaking

We don’t consider SPI part of swift-syntax’s public API and can break it as much as we want. But `swift package diagnose-api-breaking-changes` reports them as breaking changes (https://github.com/swiftlang/swift/issues/79990). This generates too much noise for the API breakage check to be useful for swift-syntax.