tools/lib/lockdep: Add empty nmi.h

Required since:

  88f1c87de11a8 ("locking/lockdep: Avoid triggering hardlockup from debug_show_all_locks()")

Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sasha Levin <Alexander.Levin@microsoft.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180813190527.16853-3-alexander.levin@microsoft.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>