fix(lwip): forward-declare struct dhcp before DHCP extra-option hook (IDFGH-17738)

[TeTeHacko]

Problem

When CONFIG_LWIP_HOOK_DHCP_EXTRA_OPTION_CUSTOM (or ..._DEFAULT) is
enabled, components/lwip/port/include/lwip_default_hooks.h declares:

void lwip_dhcp_on_extra_option(struct dhcp *dhcp, ...);

but struct dhcp is only forward-declared further down, inside the
#ifdef CONFIG_LWIP_IPV4 block — after this prototype. Every
translation unit that includes the header therefore declares struct dhcp
inside the parameter list, which GCC reports as:

error: 'struct dhcp' declared inside parameter list will not be visible
outside of this definition or declaration [-Werror]

The lwip component is built with -Werror, and this particular GCC
diagnostic has no individual -W flag, so it cannot be waived with
-Wno-error=.... The result: any project that enables the DHCP
extra-option hook fails to build.

Reproduction

• ESP-IDF v6.0.1 and current master (GCC 13.2, xtensa-esp32s3)
• CONFIG_LWIP_HOOK_DHCP_EXTRA_OPTION_CUSTOM=y with an app-provided
  lwip_dhcp_on_extra_option() implementation

Fix

Add a struct dhcp; forward declaration ahead of the prototype. A pointer
parameter only needs an incomplete type, so this is minimal and
sufficient. The redundant forward declaration inside CONFIG_LWIP_IPV4
is left untouched to keep the diff to a single added line.

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