treewide: convert GPLv2+ license info to SPDX tags

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Source/Core/InputCommon/ControllerInterface/DInput/DInput.cpp

@@ -1,6 +1,5 @@
 // Copyright 2010 Dolphin Emulator Project
-// Licensed under GPLv2+
-// Refer to the license.txt file included.
+// SPDX-License-Identifier: GPL-2.0-or-later
 
 #include "InputCommon/ControllerInterface/DInput/DInput.h"