Add realChroot for non linux/windows

3029e765e2 broke compilation on
non-Linux/Windows systems.
This change fixes that.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

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)
+}