Fix a warning in the SDK overlay
Fix a warning in the SDK overlay

This change was committed and reverted multiple times over the past
month, but now it is safe since we have bumped the minimum required
SDKs.

Swift SVN r29269

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation