lockup_detector: Make BOOTPARAM_SOFTLOCKUP_PANIC depend on LOCKUP_DETECTOR

Panic on softlockups was still depending on the softlockup detector.
But the latter has been merged into the lockup detector now.

Let's update this config dependency.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Eric Paris <eparis@redhat.com>
Cc: Randy Dunlap <randy.dunlap@oracle.com>