fix build on ILP32

Index: tests/test-opt.cpp
--- tests/test-opt.cpp.orig
+++ tests/test-opt.cpp
@@ -98,7 +98,7 @@ static helper_ctx_data helper_get_ctx_data(
     struct ggml_context * ctx_compute;
     {
         struct ggml_init_params params = {
-            /*.mem_size   =*/ (2*ndata + 2)*ggml_tensor_overhead(),
+            /*.mem_size   =*/ static_cast<size_t>((2*ndata + 2)*ggml_tensor_overhead()),
             /*.mem_buffer =*/ nullptr,
             /*.no_alloc   =*/ true,
         };