keep postcss file in src/optimize until we can remove legacy optimizer

src/optimize/base_optimizer.js

@@ -34,7 +34,7 @@ import { IS_KIBANA_DISTRIBUTABLE } from '../legacy/utils';
 import { fromRoot } from '../core/server/utils';
 import { PUBLIC_PATH_PLACEHOLDER } from './public_path_placeholder';
 
-const POSTCSS_CONFIG_PATH = require.resolve('@kbn/optimizer/postcss.config.js');
+const POSTCSS_CONFIG_PATH = require.resolve('./postcss.config.js');
 const BABEL_PRESET_PATH = require.resolve('@kbn/babel-preset/webpack_preset');
 const ISTANBUL_PRESET_PATH = require.resolve('@kbn/babel-preset/istanbul_preset');
 const EMPTY_MODULE_PATH = require.resolve('./intentionally_empty_module.js');

src/optimize/postcss.config.js

@@ -0,0 +1,22 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you 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.
+ */
+
+module.exports = {
+  plugins: [require('autoprefixer')()],
+};