openrisc/traps: Declare unhandled_exception for asmlinkage

Noticed this when workin on warnings.  As unhandled_exception is used in
entry.S we should attribute it with asmlinkage.
Signed-off-by: Stafford Horne <shorne@gmail.com>

arch/openrisc/kernel/traps.c

@@ -235,7 +235,7 @@ void __noreturn die(const char *str, struct pt_regs *regs, long err)
 	make_task_dead(SIGSEGV);
 }
 
-void unhandled_exception(struct pt_regs *regs, int ea, int vector)
+asmlinkage void unhandled_exception(struct pt_regs *regs, int ea, int vector)
 {
 	printk("Unable to handle exception at EA =0x%x, vector 0x%x",
 	       ea, vector);