irqchip/loongson-eiointc: Fix a build warning

Make acpi_get_vec_parent() be a static function, to avoid:
drivers/irqchip/irq-loongson-eiointc.c:289:20: warning: no previous prototype for 'acpi_get_vec_parent'

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220804025457.213979-1-chenhuacai@loongson.cn