win: remove dep on GetQueuedCompletionStatusEx

Libuv already works without that API since commit 153ea114ff but still
had it as a hard requirement in the import table. This code uses the
`pGetQueuedCompletionStatusEx` function pointer instead, hence it also
works on systems that don't export `GetQueuedCompletionStatusEx`.

This simple fix improves compatibility of libuv with ReactOS and
Windows XP (latter using Vista+ compatibility libraries like
https://github.com/MyTDT-Mysoft/DllCompat)

PR-URL: https://github.com/libuv/libuv/pull/2800
Reviewed-By: Bartosz Sosnowski <bartosz@janeasystems.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

src/win/core.c

@@ -449,12 +449,12 @@ static void uv__poll(uv_loop_t* loop, DWORD timeout) {
   timeout_time = loop->time + timeout;
 
   for (repeat = 0; ; repeat++) {
-    success = GetQueuedCompletionStatusEx(loop->iocp,
-                                          overlappeds,
-                                          ARRAY_SIZE(overlappeds),
-                                          &count,
-                                          timeout,
-                                          FALSE);
+    success = pGetQueuedCompletionStatusEx(loop->iocp,
+                                           overlappeds,
+                                           ARRAY_SIZE(overlappeds),
+                                           &count,
+                                           timeout,
+                                           FALSE);
 
     if (success) {
       for (i = 0; i < count; i++) {