Use #pragma once

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

include/window.h

@@ -7,8 +7,7 @@
  * window.c: Updates window attributes (X11 hints/properties).
  *
  */
-#ifndef I3_WINDOW_H
-#define I3_WINDOW_H
+#pragma once
 
 /**
  * Updates the WM_CLASS (consisting of the class and instance) for the
@@ -62,5 +61,3 @@ void window_update_role(i3Window *win, xcb_get_property_reply_t *prop, bool befo
  *
  */
 void window_update_hints(i3Window *win, xcb_get_property_reply_t *prop, bool *urgency_hint);
-
-#endif