watchdog/softlockup: Fix the SOFTLOCKUP_DETECTOR=n build

I got confused by all the various CONFIG options here about and
conflated CONFIG_LOCKUP_DETECTOR and CONFIG_SOFTLOCKUP_DETECTOR.

This results in a build failure for:

   CONFIG_LOCKUP_DETECTOR=y && CONFIG_SOFTLOCKUP_DETECTOR=n

As reported by Abdul.

Reported-and-tested-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-next <linux-next@vger.kernel.org>
Cc: linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Cc: mpe <mpe@ellerman.id.au>
Cc: sachinp <sachinp@linux.vnet.ibm.com>
Cc: stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 9cf57731

 ("watchdog/softlockup: Replace "watchdog/%u" threads with cpu_stop_work")
Link: http://lkml.kernel.org/r/20180710114210.GI2476@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>