um: Add irq chip um/mask handlers

These handlers are not optional and need in our case
dummy implementions to avoid NULL pointer bugs within
the irq core code.
Reported-and-tested-by: Toralf Foester <toralf.foerster@gmx.de>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/irq.c

@@ -337,6 +337,8 @@ static struct irq_chip normal_irq_type = {
 	.irq_disable = dummy,
 	.irq_enable = dummy,
 	.irq_ack = dummy,
+	.irq_mask = dummy,
+	.irq_unmask = dummy,
 };
 
 static struct irq_chip SIGVTALRM_irq_type = {
@@ -344,6 +346,8 @@ static struct irq_chip SIGVTALRM_irq_type = {
 	.irq_disable = dummy,
 	.irq_enable = dummy,
 	.irq_ack = dummy,
+	.irq_mask = dummy,
+	.irq_unmask = dummy,
 };
 
 void __init init_IRQ(void)