lockup detector: Fix reference to the non-existent CONFIG_DETECT_SOFTLOCKUP option

Replace CONFIG_DETECT_SOFTLOCKUP with CONFIG_LOCKUP_DETECTOR.

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Cc: fweisbec@gmail.com
Cc: a.p.zijlstra@chello.nl
Link: http://lkml.kernel.org/r/20110705133240.25e81a7a@kryten
Signed-off-by: Ingo Molnar <mingo@elte.hu>