aufs: bugfix, prototype declaration of au_cpup_xattr()

I was not aware since I always set CONFIG_AUFS_XATTR enabled.

Reported-by: brent timothy saner <brent.saner@gmail.com>
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
Merge remote-tracking branch 'aufs3-standalone/aufs3.9/00base' into aufs3.9/10static
Merge branch 'aufs3.9/10static' into aufs3.9/11proc_map
Merge branch 'aufs3.9/11proc_map' into aufs3.9/18public