test_qwen3_dflash2_gguf.cpp does not compile under clang-20: ::getpid() used without <unistd.h>

[lu-zero]

Defect

tests/vllm/models/test_qwen3_dflash2_gguf.cpp:547 calls ::getpid() but the file never
includes <unistd.h>:

tests/vllm/models/test_qwen3_dflash2_gguf.cpp:547:53: error: no member named 'getpid' in the global namespace; did you mean 'getpt'?

The include block (:51-72) has the C++ headers, nlohmann/json.hpp, gguf_builder.h and the
model headers — no POSIX header, so glibc never leaks getpid in under clang-20 + this glibc.
The file landed on main in the SPEC-DFLASH2 GGUF drafter arm (5702d8f83 /
141402e6c, PRs #1531/#1537 area) and fails to compile as soon as any branch merges current
main (verified byte-identical to origin/main e2a9e035d in the #1498 worktree; the merge
itself is clean, the TU is main's).

Sibling tests use a plain #include <unistd.h> (test_kimi_linear_paged.cpp:44,
test_loader_unaligned_offsets.cpp:46, test_ltx2_loader.cpp:26,
test_moe_async_device_ids.cpp:80) — no MSVC guard, this suite is POSIX-only.

Impact

ninja in a TT-configured worktree (clang-20 toolchain on the Blackhole P150 box) stops at
tests/CMakeFiles/test_qwen3_dflash2_gguf.dir, which blocks rebuilding ANY target after a
merge of main — including the test_qwen3_paged_engine / test_tenstorrent_backend gates
the Tenstorrent rows need. Any main-merging branch on a clang host is affected.

Ownership

Found while merging main into row/BACKEND-TENSTORRENT-HOST-FREE-F1476 (#1498); fixed
in-flow on that branch with the one-line missing include.