riscv: stacktrace: Fix undefined reference to `walk_stackframe'

Drop static declaration to fix following build error if FRAME_POINTER disabled,
  riscv64-linux-ld: arch/riscv/kernel/perf_callchain.o: in function `.L0':
  perf_callchain.c:(.text+0x2b8): undefined reference to `walk_stackframe'

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/stacktrace.c

@@ -65,7 +65,7 @@ void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,
 
 #else /* !CONFIG_FRAME_POINTER */
 
-static void notrace walk_stackframe(struct task_struct *task,
+void notrace walk_stackframe(struct task_struct *task,
 	struct pt_regs *regs, bool (*fn)(unsigned long, void *), void *arg)
 {
 	unsigned long sp, pc;