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/common.h

@@ -1,9 +1,12 @@
+// SPDX-License-Identifier: MIT
 /*
  * Compton - a compositor for X11
  *
  * Based on `xcompmgr` - Copyright (c) 2003, Keith Packard
  *
  * Copyright (c) 2011-2013, Christopher Jeffrey
+ * Copyright (c) 2018, Yuxuan Shui <yshuiv7@gmail.com>
+ *
  * See LICENSE for more information.
  *
  */