arm64: Fix compilation error on UP builds

In file included from ./arch/arm64/include/asm/irq_work.h:4:0,
        from include/linux/irq_work.h:46,
        from include/linux/perf_event.h:49,
        from include/linux/ftrace_event.h:9,
        from include/trace/syscall.h:6,
        from include/linux/syscalls.h:81,
        from init/main.c:18:
./arch/arm64/include/asm/smp.h:24:3:
        error: #error "<asm/smp.h> included in non-SMP build"
 # error "<asm/smp.h> included in non-SMP build"
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Fixes: 36310736 ("arm64: Tell irq work about self IPI support")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>

arch/arm64/include/asm/irq_work.h

 #ifndef __ASM_IRQ_WORK_H
 #define __ASM_IRQ_WORK_H
 
+#ifdef CONFIG_SMP
+
 #include <asm/smp.h>
 
 static inline bool arch_irq_work_has_interrupt(void)
@@ -8,4 +10,13 @@ static inline bool arch_irq_work_has_interrupt(void)
 	return !!__smp_cross_call;
 }
 
+#else
+
+static inline bool arch_irq_work_has_interrupt(void)
+{
+	return false;
+}
+
+#endif
+
 #endif /* __ASM_IRQ_WORK_H */