x86: Fix symbol annotation for arch/x86/lib/clear_page_64.S::clear_page_c

Noticed the zero-sized function symbol while looking at 'perf' profiles,
it causes the profiler to display those addresses in hexa.

Turns out that this was wrong/bogus for an eternity.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Acked-by: Alexander van Heukelum <heukelum@fastmail.fm>
Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>
LKML-Reference: <1246366820.6538.1.camel@marge.simson.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>