win32: move some code into #ifdef G_OS_UNIX

Fix various bits of code/declarations that are only used by G_OS_UNIX
but were still visible to G_OS_WIN32.

https://bugzilla.gnome.org/show_bug.cgi?id=688109

gio/gdbusprivate.c

@@ -963,6 +963,7 @@ write_message_async_cb (GObject      *source_object,
  * write-lock is not held on entry
  * output_pending is PENDING_WRITE on entry
  */
+#ifdef G_OS_UNIX
 static gboolean
 on_socket_ready (GSocket      *socket,
                  GIOCondition  condition,
@@ -972,6 +973,7 @@ on_socket_ready (GSocket      *socket,
   write_message_continue_writing (data);
   return FALSE; /* remove source */
 }
+#endif
 
 /* called in private thread shared by all GDBusConnection instances
  *
@@ -982,15 +984,17 @@ static void
 write_message_continue_writing (MessageToWriteData *data)
 {
   GOutputStream *ostream;
-  GSimpleAsyncResult *simple;
 #ifdef G_OS_UNIX
+  GSimpleAsyncResult *simple;
   GUnixFDList *fd_list;
 #endif
 
+#ifdef G_OS_UNIX
   /* Note: we can't access data->simple after calling g_async_result_complete () because the
    * callback can free @data and we're not completing in idle. So use a copy of the pointer.
    */
   simple = data->simple;
+#endif
 
   ostream = g_io_stream_get_output_stream (data->worker->stream);
 #ifdef G_OS_UNIX
@@ -1106,7 +1110,9 @@ write_message_continue_writing (MessageToWriteData *data)
                                    write_message_async_cb,
                                    data);
     }
+#ifdef G_OS_UNIX
  out:
+#endif
   ;
 }