license: add copyright line for all contributors

Recently it has come up that someone wants to re-use part of
wlroots in their own project. The standard procedure is to copy
over the license text and copyright lines.

Let's make it clear that the wlroots copyright is held by all
wlroots contributors, and that the license file comes from the
wlroots project.

See [1] for more details.

[1]: https://www.linuxfoundation.org/blog/blog/copyright-notices-in-open-source-software-projects

LICENSE

@@ -1,5 +1,6 @@
 Copyright (c) 2017, 2018 Drew DeVault
 Copyright (c) 2014 Jari Vetoniemi
+Copyright (c) 2023 The wlroots contributors
 
 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