Add realChroot for non linux/windows

3029e765e241ea2b5249868705dbf9095bc4d529 broke compilation on
non-Linux/Windows systems.
This change fixes that.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>
(cherry picked from commit 34d5b8867fe83403a6998d043a32a49e087f2477)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/chrootarchive/chroot_unix.go

@@ -10,3 +10,7 @@ func chroot(path string) error {
 	}
 	return unix.Chdir("/")
 }
+
+func realChroot(path string) error {
+	return chroot(path)
+}