irqchip/loongson-liointc: Fix build warnings

Fix build warnings as below:

>> drivers/irqchip/irq-loongson-liointc.c:134:12: warning: no previous prototype for 'liointc_of_init' [-Wmissing-prototypes]
     134 | int __init liointc_of_init(struct device_node *node,
         |            ^~~~~~~~~~~~~~~

Fixes: dbb15226

 ("irqchip: Add driver for Loongson I/O Local Interrupt Controller")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Huacai Chen <chenhuacai@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210105025651.772024-1-chenhuacai@loongson.cn