Use (void) instead of () for functions without args (Thanks fernandotcl)

See also:
http://article.gmane.org/gmane.linux.kernel/1268792

The C compiler will handle (void) as "no arguments" and () as "variadic
function" (equivalent to (...)) which might lead to subtle errors, such
as the one which was fixed with commit 0ea64ae4.

include/handlers.h

@@ -43,7 +43,7 @@ void handle_event(int type, xcb_generic_event_t *event);
  * received from X11
  *
  */
-void property_handlers_init();
+void property_handlers_init(void);
 
 #if 0
 /**