tools/liblockdep: Correct macro for WARN

tools/liblockdep: Correct macro for WARN

As Peter Zijlstra pointed out, the varargs for WARN() are
optional, so we need to correctly handle the case where they
don't exist.

This would cause a compilation error.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1440479985-6696-2-git-send-email-sasha.levin@oracle.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>