Sort out license problems

IANAL, but I think I am allowed to add missing copyright notices for
someone else. And I did my best job using git history to figure out who
wrote which functions. So I hope everything is fine.

src/compton.h

@@ -1,6 +1,5 @@
-/**
- * compton.h
- */
+// SPDX-License-Identifier: MIT
+// Copyright (c)
 
 // Throw everything in here.