irqchip/gic-v3: Fix 'broken_rdists' unused warning when !SMP and !ACPI

Compiling the GICv3 driver on arm32 with CONFIG_SMP disabled
(CONFIG_ACPI is not available) generates an unused variable warning for
'broken_rdists'. Add a __maybe_unused attribute to silence the compiler.

Fixes: d633da5d ("irqchip/gic-v3: Add support for ACPI's disabled but 'online capable' CPUs")
Cc: <stable@vger.kernel.org> # .x
Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>