lguest: fix timer interrupt setup

Without an IRQ chip set, we now get a WARN_ON and no timer interrupt.  This
prevents booting.

Fortunately, the fix is a one-liner: set up the timer IRQ like everything
else.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: stable@kernel.org # .39.x

arch/x86/lguest/boot.c

@@ -993,6 +993,7 @@ static void lguest_time_irq(unsigned int irq, struct irq_desc *desc)
 static void lguest_time_init(void)
 {
 	/* Set up the timer interrupt (0) to go to our simple timer routine */
+	lguest_setup_irq(0);
 	irq_set_handler(0, lguest_time_irq);
 
 	clocksource_register_hz(&lguest_clock, NSEC_PER_SEC);