vendor: add buildkit

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/github.com/tonistiigi/fsutil/chtimes_nolinux.go

@@ -0,0 +1,20 @@
+// +build !linux
+
+package fsutil
+
+import (
+	"os"
+	"time"
+)
+
+func chtimes(path string, un int64) error {
+	mtime := time.Unix(0, un)
+	fi, err := os.Lstat(path)
+	if err != nil {
+		return err
+	}
+	if fi.Mode()&os.ModeSymlink != 0 {
+		return nil
+	}
+	return os.Chtimes(path, mtime, mtime)
+}