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

arch/m68k/include/asm/mac_via.h

@@ -254,6 +254,8 @@
 extern volatile __u8 *via1,*via2;
 extern int rbv_present,via_alt_mapping;
 
+struct irq_desc;
+
 extern void via_register_interrupts(void);
 extern void via_irq_enable(int);
 extern void via_irq_disable(int);