Convert PolicyConfig licence header to SPDX

I don't have the contact email of the original author and it is not easy to trace back to the source because the project has been split and forked

plugins/systemvolume/PolicyConfig.h

@@ -1,25 +1,8 @@
-/* The MIT License (MIT)
+/*
+ * SPDX-License-Identifier: MIT
+ * SPDX-FileCopyrightText: 2015 EreTIk
+ */
 
-Copyright (c) 2015 DefSound
-
-Permission is hereby granted, free of charge, to any person obtaining a copy
-of this software and associated documentation files (the "Software"), to deal
-in the Software without restriction, including without limitation the rights
-to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-copies of the Software, and to permit persons to whom the Software is
-furnished to do so, subject to the following conditions:
-
-The above copyright notice and this permission notice shall be included in
-all copies or substantial portions of the Software.
-
-THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
-THE SOFTWARE.
-*/
 // ----------------------------------------------------------------------------
 // PolicyConfig.h
 // Undocumented COM-interface IPolicyConfig.