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/manage.h

@@ -27,7 +27,7 @@ void manage_existing_windows(xcb_window_t root);
  * side-effects which are to be expected when continuing to run i3.
  *
  */
-void restore_geometry();
+void restore_geometry(void);
 
 /**
  * Do some sanity checks and then reparent the window.