x86: Add missing annotation to arch/x86/lib/copy_user_64.S::copy_to_user

While examining symbol generation in perf_counter tools, I
noticed that copy_to_user() had no size in vmlinux's symtab.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Acked-by: Alexander van Heukelum <heukelum@fastmail.fm>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
LKML-Reference: <1246512440.13293.3.camel@marge.simson.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>