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>