bpf: change bpf_alu_sign_string and bpf_movsx_string to static

bpf: change bpf_alu_sign_string and bpf_movsx_string to static

The bpf_alu_sign_string and bpf_movsx_string introduced in commit
f835bb622299 ("bpf: Add kernel/bpftool asm support for new instructions")
are only used in disasm.c now, change them to static.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202308050615.wxAn1v2J-lkp@intel.com/
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230803023128.3753323-1-yangyingliang@huawei.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>