m68k: Fix build warning in mac_via.h

m68k: Fix build warning in mac_via.h

Fengguang Wu's kbuild test robot reported the following new m68k warnings:

     In file included from drivers/nubus/nubus.c:22:0:
  >> arch/m68k/include/asm/mac_via.h:262:47: warning: 'struct irq_desc' declared inside parameter list [enabled by default]
  >> arch/m68k/include/asm/mac_via.h:262:47: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]

Caused by the reworking of the generic local_bh{dis,en}able() code.

To fix it, forward declare 'struct irq_desc'.

Reported-by: Fengguang Wu <[email protected]>
Fixes: c795eb55e740 ("sched/preempt, locking: Rework local_bh_{dis,en}able()")
Signed-off-by: Peter Zijlstra <[email protected]>
Acked-by: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>