arm64: cpuidle: fix #ifdef for acpi functions

The acpi_processor_ffh_lpi_* functions are defined whenever CONFIG_ACPI
is enabled, but only called and declared when CONFIG_ACPI_PROCESSOR_IDLE
is also enabled. Without that, a W=1 build triggers missing-prototope
warnings, so the #ifdef needs to be adapted:

arch/arm64/kernel/cpuidle.c:60:5: error: no previous prototype for 'acpi_processor_ffh_lpi_probe' [-Werror=missing-prototypes]
arch/arm64/kernel/cpuidle.c:65:15: error: no previous prototype for 'acpi_processor_ffh_lpi_enter' [-Werror=missing-prototypes]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20230516160642.523862-8-arnd@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>