build: fix header order on mingw

In file included from ../gnulib/lib/unistd.h:51:0,
                 from ../src/util/util.h:30,
                 from rpc/virkeepalive.c:29:
/usr/x86_64-w64-mingw32/sys-root/mingw/include/winsock2.h:15:2: warning: #warning Please include winsock2.h before windows.h [-Wcpp]

Reported by Marc-André Lureau.

* src/util/threads-win32.h (includes): Pick up winsock2.h before
windows.h, as required by mingw64.

src/util/threads-win32.h

 /*
  * threads-win32.h basic thread synchronization primitives
  *
- * Copyright (C) 2009, 2011 Red Hat, Inc.
+ * Copyright (C) 2009, 2011-2012 Red Hat, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -21,6 +21,9 @@
 
 #include "internal.h"
 
+#ifdef HAVE_WINSOCK2_H
+# include <winsock2.h>
+#endif
 #include <windows.h>
 
 struct virMutex {