arm64: Declare ACPI parking protocol CPU operation if needed

It's obvious we needn't declare the corresponding CPU operation when
CONFIG_ARM64_ACPI_PARKING_PROTOCOL is disabled, even it doesn't cause
any compiling warnings.
Signed-off-by: Gavin Shan <gshan@redhat.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/cpu_ops.c

@@ -15,7 +15,9 @@
 #include <asm/smp_plat.h>
 
 extern const struct cpu_operations smp_spin_table_ops;
+#ifdef CONFIG_ARM64_ACPI_PARKING_PROTOCOL
 extern const struct cpu_operations acpi_parking_protocol_ops;
+#endif
 extern const struct cpu_operations cpu_psci_ops;
 
 const struct cpu_operations *cpu_ops[NR_CPUS] __ro_after_init;