rtweekend.h: add standard header blurb

Author: hollasch

src/common/rtweekend.h

@@ -1,5 +1,13 @@
 #ifndef RTWEEKEND_H
 #define RTWEEKEND_H
+//==============================================================================================
+// To the extent possible under law, the author(s) have dedicated all copyright and related and
+// neighboring rights to this software to the public domain worldwide. This software is
+// distributed without any warranty.
+//
+// You should have received a copy (see file COPYING.txt) of the CC0 Public Domain Dedication
+// along with this software. If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
+//==============================================================================================
 
 #include <cstdlib>
 #include <limits>