Use #pragma once

#pragma once is safer and simpler. According to Wikipedia it's supported by all major compilers.

include/randr.h

@@ -9,8 +9,7 @@
  * (take your time to read it completely, it answers all questions).
  *
  */
-#ifndef I3_RANDR_H
-#define I3_RANDR_H
+#pragma once
 
 #include "data.h"
 #include <xcb/randr.h>
@@ -121,5 +120,3 @@ Output *get_output_next(direction_t direction, Output *current, output_close_far
  *
  */
 Output *get_output_next_wrap(direction_t direction, Output *current);
-
-#endif