build failure on !linux !windows platforms

Description

#39292 broke compilation on !linux !windows build tags;

Line 135 in 70f67c6

if err := realChroot(root); err != nil {

(build tag !windows) uses realChroot, defined here:

Line 105 in 70f67c6

func realChroot(path string) error {

(implicit build tag linux)

I'm not sure I understand the nature of the security fix well enough to fix this myself.
Steps to reproduce the issue:

Describe the results you received:
github.com/docker/docker/pkg/chrootarchive/archive_unix.go:135:12: undefined: realChroot

Describe the results you expected:
working build :)

Additional information you deem important (e.g. issue happens only occasionally):
Should the call have been to chroot, not realChroot?

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