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/commands.c

@@ -108,7 +108,7 @@ static void nagbar_cleanup(EV_P_ ev_cleanup *watcher, int revent) {
 }
 #endif
 
-void cmd_MIGRATION_start_nagbar() {
+void cmd_MIGRATION_start_nagbar(void) {
     if (migration_pid != -1) {
         fprintf(stderr, "i3-nagbar already running.\n");
         return;