windows: guard static_assert with __STDC_VERSION__ for C99 compatibility

[Youw]

Problem

windows/hidapi_descriptor_reconstruct.h:127 uses static_assert to sanity-check the in-memory layout of hid_pp_link_collection_node_:

static_assert(sizeof(struct hid_pp_link_collection_node_) == 16, ...);

static_assert was added to <assert.h> in C11. Building with -std=c99 — for example via set(CMAKE_C_STANDARD 99) in a parent CMake project, as reported in #764 — fails at that line on mingw/gcc:

windows/hidapi_descriptor_reconstruct.h:127:15: error: expected declaration specifiers or '...' before 'sizeof'

Change

The assertion's preceding comment describes it as a "risk-reduction-measure" rather than a strict requirement. Guard it:

#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
static_assert(...);
#endif

• C11 / C17 / C23 toolchains still get the compile-time check (behaviour unchanged).
• C99 and earlier skip it and build cleanly.
• MSVC in its default C mode (where __STDC_VERSION__ is typically not set that high) also skips it — a minor reduction in the safety net there, but the production build is unaffected.

Net diff: +4 / -0 in one file.

Verification

Reproduced and fixed locally with x86_64-w64-mingw32-gcc (GCC 10, MinGW-w64) in WSL, compiling the real backend sources:

build	-std=c99	-std=c11
master (no fix)	fail — error: expected declaration specifiers or '...' before 'sizeof' at L127	pass
this PR	pass	pass

Tested with both windows/hid.c and windows/hidapi_descriptor_reconstruct.c (the two sources that include the affected header).

Closes #764.

Drafted with Claude Code.