metag/asm/irq.h: Declare init_IRQ

Fixes the following sparce warning:

warning: symbol 'init_IRQ' was not declared. Should it be static?
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: James Hogan <james.hogan@imgtec.com>

arch/metag/include/asm/irq.h

@@ -17,6 +17,7 @@ struct pt_regs;
 
 int tbisig_map(unsigned int hw);
 extern void do_IRQ(int irq, struct pt_regs *regs);
+extern void init_IRQ(void);
 
 #ifdef CONFIG_METAG_SUSPEND_MEM
 int traps_save_context(void);