Fix CI build failure

See https://github.com/ggml-org/llama.cpp/actions/runs/16798370266/job/47573716079?pr=15132#step:7:486
Building with VS generator worked though.

Author: ORippler

ggml/src/ggml-cuda/mean.cu

@@ -16,7 +16,7 @@ void ggml_cuda_op_mean(ggml_backend_cuda_context & ctx, ggml_tensor * dst) {
 
     const dim3 block_nums(nrows, 1, 1);
     if ((nrows / ctx.sm_count) < 2) {
-        constexpr dim3 block_dims(512, 1, 1);
+        const dim3 block_dims(512, 1, 1);
         reduce_rows_f32</*norm=*/true><<<block_nums, block_dims, 0, stream>>>(src0_d, dst_d, ncols);
     } else {
         const dim3 block_dims(ncols < 1024 ? 32 : 128, 1, 1);