MIPS: IRQ: Add prototype for function init_IRQ

This commit adds a prototype to fix error at W=1:

arch/mips/kernel/irq.c:52:13: error: no previous prototype for
'init_IRQ' [-Werror=missing-prototypes]

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Huacai Chen <chenhuacai@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>