riscv: ptrace: Fix no prototype warnings

Fix the following W=1 kernel compilation warnings:
arch/riscv/kernel/ftrace.c:186:6: warning: no previous prototype for ‘prepare_ftrace_return’ [-Wmissing-prototypes]
  186 | void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr,
      |      ^~~~~~~~~~~~~~~~~~~~~
arch/riscv/kernel/ptrace.c:239:15: warning: no previous prototype for ‘do_syscall_trace_enter’ [-Wmissing-prototypes]
  239 | __visible int do_syscall_trace_enter(struct pt_regs *regs)
      |               ^~~~~~~~~~~~~~~~~~~~~~
arch/riscv/kernel/ptrace.c:262:16: warning: no previous prototype for ‘do_syscall_trace_exit’ [-Wmissing-prototypes]
  262 | __visible void do_syscall_trace_exit(struct pt_regs *regs)
      |                ^~~~~~~~~~~~~~~~~~~~~

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