riscv: process: Fix no prototype for arch_dup_task_struct

Fix the following W=1 compilation warning:
arch/riscv/kernel/process.c:114:5: warning: no previous prototype for ‘arch_dup_task_struct’ [-Wmissing-prototypes]
  114 | int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
      |     ^~~~~~~~~~~~~~~~~~~~

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