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/Common/MD5.h

@@ -1,6 +1,5 @@
 // Copyright 2016 Dolphin Emulator Project
-// Licensed under GPLv2+
-// Refer to the license.txt file included.
+// SPDX-License-Identifier: GPL-2.0-or-later
 
 #pragma once
 
@@ -10,4 +9,4 @@
 namespace MD5
 {
 std::string MD5Sum(const std::string& file_name, std::function<bool(int)> progress);
-}
+}