Support Wayland protocol .xml in license check

As seen in https://codereview.qt-project.org/c/qt/qtqa/+/381966 we have to add exclusions to the license check whenever new protocols are added to examples in Qt Wayland, since the script does not recognize the license headers in these files. In addition, since there is a catchall exclusion, new protocols added to extensions will never be checked for license header at all.

The files are easily parsable as XML and the licenses are embedded inside a copyright-tag:

<copyright>
 Copyright (C) 2021 The Qt Company Ltd.
	
 Contact: http://www.qt.io/licensing/
		
 This file is part of the examples of the Qt Wayland module.
$QT_BEGIN_LICENSE:BSD$
[...]
</copyright>

It would be useful if the script could recognize these licenses so that we didn't need the exclusion patterns.