Add GN Build Files and Custom Modifications

Issue:https://gitee.com/openharmony/build/issues/I6UFTP
Signed-off-by: lubinglun <lubinglun@huawei.com>

BUILD.gn

@@ -0,0 +1,46 @@
+# Copyright (c) 2023 Huawei Device Co., Ltd.
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import("//build/ohos.gni")
+
+ohos_cargo_crate("lib") {
+  crate_name = "syn"
+  crate_type = "rlib"
+  crate_root = "src/lib.rs"
+
+  sources = [ "src/lib.rs" ]
+  edition = "2018"
+  cargo_pkg_version = "1.0.107"
+  cargo_pkg_authors = "David Tolnay <dtolnay@gmail.com>"
+  cargo_pkg_name = "syn"
+  cargo_pkg_description = "Parser for Rust source code"
+  deps = [
+    "//third_party/rust/crates/proc-macro2:lib",
+    "//third_party/rust/crates/quote:lib",
+    "//third_party/rust/crates/unicode-ident:lib",
+  ]
+  features = [
+    "clone-impls",
+    "derive",
+    "extra-traits",
+    "full",
+    "parsing",
+    "printing",
+    "proc-macro",
+    "quote",
+    "visit",
+    "visit-mut",
+  ]
+  build_root = "build.rs"
+  build_sources = [ "build.rs" ]
+}