cmake : skip PCH for llama-server PCH when using MSVC (#28763)
This commit fixes an issue that I introduced when adding PCH
(precompiled headers) in Commit 3bcfeb700fce9ff38a050dcd3f6a856319e948ba
("cmake : add PCH and unity build to improve build times (#28091)".
See linked issue for details.
Co-authored-by: mjungnickel18
Co-authored-by: Pascal <admin@serveurperso.com>
Resolves: https://github.com/ggml-org/llama.cpp/issues/28758
Refs: https://github.com/ggml-org/llama.cpp/actions/runs/34592933983/job/103262608990#step:9:1284