riscv: process: Fix no prototype for show_regs

Include header file to fix the following W=1 compilation warning:
arch/riscv/kernel/process.c:78:6: warning: no previous prototype for ‘show_regs’ [-Wmissing-prototypes]
   78 | void show_regs(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>

arch/riscv/kernel/process.c

@@ -10,6 +10,7 @@
 #include <linux/cpu.h>
 #include <linux/kernel.h>
 #include <linux/sched.h>
+#include <linux/sched/debug.h>
 #include <linux/sched/task_stack.h>
 #include <linux/tick.h>
 #include <linux/ptrace.h>