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>