[net][lwip-dhcpd] Add missing <string.h> include in dhcp_server_raw.c

[JHB11Hinson]

拉取/合并请求描述：(PR description)

为什么提交这份PR (why to submit this PR)

components/net/lwip-dhcpd/dhcp_server_raw.c 使用了 memcmp()、memset()、strlen()、strncmp() 和 strchr()，但没有包含 <string.h>。在 C 中这些函数会被隐式声明为返回 int：在 64 位目标上，p = strchr(...) 会把 64 位指针截断为 32 位再赋给 char *，得到错误地址，属于未定义行为；开启 -Werror=implicit-function-declaration 时该文件还会直接编译失败。

The file calls memcmp(), memset(), strlen(), strncmp() and strchr() but does not include <string.h>. They are implicitly declared as returning int; on 64-bit targets the strchr() result is truncated when assigned to a char *, which is undefined behaviour.

你的解决方案是什么 (what is your solution)

在 libc 头文件区块加入 #include <string.h>，与仓库内其它 net 组件（如 components/net/netdev/src/netdev.c、components/net/lwip/port/ethernetif.c）保持一致。

Add #include <string.h> to the libc header block, consistent with other net components in the tree.

请提供验证的bsp和config (provide the config and bsp)

• BSP: bsp/w60x（仅用于编译验证；该 BSP 使能 RT_USING_LWIP + LWIP_USING_DHCPD，对应 dhcp_server_raw.c 分支）
• .config: RT_USING_LWIP、LWIP_USING_DHCPD（lwip 2.0.3）
• action: 本地使用 bsp/w60x/rtconfig.h + lwip-2.0.3 头文件，对 dhcp_server_raw.c 单文件做编译验证（host gcc，-Werror=implicit-function-declaration）：修复前 10 处 implicit declaration、退出码 1；修复后 0 处、退出码 0。未做完整板级构建，CI 结果为准。

当前拉取/合并请求的状态 Intent for your PR

必须选择一项 Choose one (Mandatory):

• 本拉取/合并请求是一个草稿版本 This PR is for a code-review and is intended to get feedback
• 本拉取/合并请求是一个成熟版本 This PR is mature, and ready to be integrated into the repo

代码质量 Code Quality：

我在这个拉取/合并请求中已经考虑了 As part of this pull request, I've considered the following:

• 已经仔细查看过代码改动的对比 Already check the difference between PR and old code
• 代码风格正确，包括缩进空格，命名及其他风格 Style guide is adhered to, including spacing, naming and other styles
• 没有垃圾代码，代码尽量精简，不包含#if 0代码，不包含已经被注释了的代码 All redundant code is removed and cleaned up
• 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或BSP All modifications are justified and not affect other components or BSP
• 对难懂代码均提供对应的注释 I've commented appropriately where code is tricky
• 代码是高质量的 Code in this PR is of high quality
• 已经使用clang-format 源码格式化工具确保格式符合RT-Thread代码规范 This PR has been formatted with clang-format and complies with RT-Thread code specification
• 如果是新增bsp, 已经添加ci检查到.github/ALL_BSP_COMPILE.json 详细请参考链接BSP自查

[CLAassistant]

All committers have signed the CLA.

[github-actions]

👋 感谢您对 RT-Thread 的贡献！Thank you for your contribution to RT-Thread!

为确保代码符合 RT-Thread 的编码规范，请在你的仓库中执行以下步骤运行代码格式化工作流（如果格式化CI运行失败）。
To ensure your code complies with RT-Thread's coding style, please run the code formatting workflow by following the steps below (If the formatting of CI fails to run).

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Once completed, commits will be pushed to the fix/lwip-dhcpd-missing-string-header branch automatically, and the related Pull Request will be updated.

如有问题欢迎联系我们，再次感谢您的贡献！💐
If you have any questions, feel free to reach out. Thanks again for your contribution!

[github-actions]

📌 Code Review Assignment

🏷️ Tag: components

Reviewers: @Maihuanyi

Changed Files (Click to expand)

• components/net/lwip-dhcpd/dhcp_server_raw.c

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📊 Current Review Status (Last Updated: 2026-09-24 21:26 CST)

• ⌛ @Maihuanyi Pending Review

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