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.

src/main.c

@@ -212,7 +212,7 @@ static void xkb_got_event(EV_P_ struct ev_io *w, int revents) {
  * Exit handler which destroys the main_loop. Will trigger cleanup handlers.
  *
  */
-static void i3_exit() {
+static void i3_exit(void) {
 /* We need ev >= 4 for the following code. Since it is not *that* important (it
  * only makes sure that there are no i3-nagbar instances left behind) we still
  * support old systems with libev 3. */