arm64/include/asm: Fixed a warning about 'struct pt_regs'

If I include asm/irq.h on the top of my code, and set ARCH=arm64,
I'll get a compile warning, details are below:
warning: ‘struct pt_regs’
declared inside parameter list [enabled by default]

This patch is suggested by Arnd, see:
http://lists.infradead.org/pipermail/linux-arm-kernel/2014-December/308270.html

Signed-off-by: Chunyan Zhang <chunyan.zhang@spreadtrum.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/irq.h

@@ -3,6 +3,8 @@
 
 #include <asm-generic/irq.h>
 
+struct pt_regs;
+
 extern void migrate_irqs(void);
 extern void set_handle_irq(void (*handle_irq)(struct pt_regs *));