x86, apic: Fix prototype in hw_irq.h

This warning:

 In file included from arch/x86/include/asm/ipi.h:23,
                  from arch/x86/kernel/apic/apic_noop.c:27:
 arch/x86/include/asm/hw_irq.h:105: warning: ‘struct irq_desc’ declared inside parameter list
 arch/x86/include/asm/hw_irq.h:105: warning: its scope is only this definition or declaration, which is probably not what you want

triggers because irq_desc is defined after hw_irq.h is included
in irq.h. Since it's pointer reference only, a forward declaration
of the type will solve the problem.

LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/include/asm/hw_irq.h

@@ -102,6 +102,8 @@ struct irq_cfg {
 extern struct irq_cfg *irq_cfg(unsigned int);
 extern int assign_irq_vector(int, struct irq_cfg *, const struct cpumask *);
 extern void send_cleanup_vector(struct irq_cfg *);
+
+struct irq_desc;
 extern unsigned int set_desc_affinity(struct irq_desc *, const struct cpumask *);
 extern int IO_APIC_get_PCI_irq_vector(int bus, int devfn, int pin, struct io_apic_irq_attr *irq_attr);
 extern void setup_ioapic_dest(void);