[build] set ONNX_USE_LITE_PROTO=ON only for wasm when using vcpkg (#26485)

### Description

Before this change, ONNX_USE_LITE_PROTO is always set to OFF for all
vcpkg builds, including web assembly build. This PR modifies the
behavior to use lite proto when building targetting wasm.

No change to non-vcpkg build.

Author: fs-eire

cmake/vcpkg-ports/onnx/portfile.cmake

@@ -36,7 +36,6 @@ vcpkg_cmake_configure(
         "-DProtobuf_PROTOC_EXECUTABLE:FILEPATH=${PROTOC}"
         -DONNX_ML=ON
         -DONNX_USE_PROTOBUF_SHARED_LIBS=${USE_PROTOBUF_SHARED}
-        -DONNX_USE_LITE_PROTO=OFF
         -DONNX_USE_MSVC_STATIC_RUNTIME=${USE_STATIC_RUNTIME}
         -DONNX_BUILD_TESTS=OFF
         -DONNX_BUILD_BENCHMARKS=OFF

tools/python/util/vcpkg_helpers.py

@@ -74,6 +74,15 @@ def add_port_configs(f, has_exception: bool, is_emscripten: bool, enable_minimal
     )"""
         )
 
+    if is_emscripten:
+        # Uses ONNX_USE_LITE_PROTO=ON for WebAssembly build.
+        f.write(
+            r"""
+    list(APPEND VCPKG_CMAKE_CONFIGURE_OPTIONS
+        "-DONNX_USE_LITE_PROTO=ON"
+    )"""
+        )
+
     f.write(r"""
 endif() # End ONNX-specific block
 """)