sync: use Event for waitgroup on windows

vlib/sync/waiter_nix.c.v

@@ -0,0 +1,22 @@
+// Copyright (c) 2019 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license
+// that can be found in the LICENSE file.
+module sync
+
+[ref_only]
+struct Waiter{
+mut:
+	mx &Mutex
+}
+
+pub fn (mut w Waiter) wait() {
+	w.mx.lock()
+}
+
+pub fn (mut w Waiter) stop() {
+	w.mx.unlock()
+}
+pub fn new_waiter() &Waiter {
+	w := &Waiter{mx: new_mutex()}
+	return w
+}