bpftool: Remove duplicate string.h header

./tools/bpf/bpftool/sign.c: string.h is included more than once.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=25502


Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Acked-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/r/20250926095240.3397539-2-jiapeng.chong@linux.alibaba.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>