riscv: irq: Fix no prototype warning

Fix the following W=1 kernel compilation warning:
arch/riscv/kernel/irq.c:19:13: warning: no previous prototype for ‘init_IRQ’ [-Wmissing-prototypes]
   19 | void __init init_IRQ(void)
      |             ^~~~~~~~

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Nanyong Sun <sunnanyong@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>