Fix windows compilation for the new chroot tar

Author: tscolari

base_image_puller/unpacker/tar.go base_image_puller/unpacker/tar_linux.gobase_image_puller/unpacker/tar.go renamed to base_image_puller/unpacker/tar_linux.go

@@ -0,0 +1,21 @@
+package unpacker // import "code.cloudfoundry.org/grootfs/base_image_puller/unpacker"
+
+import (
+	"code.cloudfoundry.org/grootfs/base_image_puller"
+	"code.cloudfoundry.org/lager"
+)
+
+type UnpackStrategy struct {
+	Name               string
+	WhiteoutDevicePath string
+}
+
+type TarUnpacker struct{}
+
+func NewTarUnpacker(unpackStrategy UnpackStrategy) (*TarUnpacker, error) {
+	panic("not implemented")
+}
+
+func (u *TarUnpacker) Unpack(logger lager.Logger, spec base_image_puller.UnpackSpec) error {
+	panic("not implemented")
+}