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 <fengguang.wu@intel.com>
Fixes: c795eb55e740 ("sched/preempt, locking: Rework local_bh_{dis,en}able()")
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Acked-by:  <geert@linux-m68k.org>
Link: http://lkml.kernel.org/r/20140112212456.GQ7572@laptop.programming.kicks-ass.net


Signed-off-by: Ingo Molnar <mingo@kernel.org>