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>