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