ci : disable OpenMP in wasm webgpu build

Upstream master's ubuntu-wasm job is red with the same error:
github.com/ggml-org/llama.cpp/actions/runs/29361861822/job/87183687962

Signed-off-by: Guilherme Gallo <guilherme.gallo@collabora.com>

Author: Guilherme Gallo

.github/workflows/build-webgpu.yml

@@ -190,6 +190,7 @@ jobs:
             -G "Ninja" \
             -DCMAKE_BUILD_TYPE=Release \
             -DGGML_WEBGPU=ON \
+            -DGGML_OPENMP=OFF \
             -DLLAMA_OPENSSL=OFF \
             -DEMDAWNWEBGPU_DIR=emdawnwebgpu_pkg