Use #pragma once

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

include/handlers.h

@@ -8,8 +8,7 @@
  *             …).
  *
  */
-#ifndef I3_HANDLERS_H
-#define I3_HANDLERS_H
+#pragma once
 
 #include <xcb/randr.h>
 
@@ -63,5 +62,3 @@ int handle_window_type(void *data, xcb_connection_t *conn, uint8_t state,
                        xcb_window_t window, xcb_atom_t atom,
                        xcb_get_property_reply_t *property);
 #endif
-
-#endif