genirq: Add missing __must_hold() sparse annotation

Add __must_hold() annotation to address the following sparse warning:

  warning: context imbalance in irq_wait_for_poll - unexpected unlock
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20191216144208.29852-2-jbi.octave@gmail.com

kernel/irq/spurious.c

@@ -34,6 +34,7 @@ static atomic_t irq_poll_active;
  * true and let the handler run.
  */
 bool irq_wait_for_poll(struct irq_desc *desc)
+	__must_hold(&desc->lock)
 {
 	if (WARN_ONCE(irq_poll_cpu == smp_processor_id(),
 		      "irq poll in progress on cpu %d for irq %d\n",