kernel/watchdog: fix Kconfig constraints for perf hardlockup watchdog

Commit 05a4a952 ("kernel/watchdog: split up config options") lost
the perf-based hardlockup detector's dependency on PERF_EVENTS, which
can result in broken builds with some powerpc configurations.

Restore the dependency.  Add it in for x86 too, despite x86 always
selecting PERF_EVENTS it seems reasonable to make the dependency
explicit.

Link: http://lkml.kernel.org/r/20170810114452.6673-1-npiggin@gmail.com
Fixes: 05a4a952

 ("kernel/watchdog: split up config options")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>