Add BUILD.gn for tinyxml

Change-Id: I82d2bb25470d46516e0ab6114bf85cb93f9023ac

system/build/secondary/third_party/tinyxml2/BUILD.gn

+#
+#  Copyright (C) 2015 Google, Inc.
+#
+#  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.
+#
+
+static_library("tinyxml2") {
+  sources = [
+    "tinyxml2.cpp"
+  ]
+
+  include_dirs = [ "." ]
+}