[DirectoryWatcher] Fix cross-compilation.
[DirectoryWatcher] Fix cross-compilation.

Some platforms have the header but don't have support for fs_event.

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang