zos: use proper prototype for epoll_init()

`int epoll_init()` declares a function that takes any number of
arguments, use `int epoll_init(void)` instead.

PR-URL: https://github.com/libuv/libuv/pull/1314
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: John Barboza <jbarboza@ca.ibm.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

src/unix/os390-syscalls.c

@@ -120,7 +120,7 @@ static void maybe_resize(uv__os390_epoll* lst, unsigned int len) {
 }
 
 
-static void epoll_init() {
+static void epoll_init(void) {
   QUEUE_INIT(&global_epoll_queue);
   if (uv_mutex_init(&global_epoll_lock))
     abort();