pulse-server: make sure we are active when draining

When we start draining, set ourselves active so that the process
function is called where the draining is completed.

This avoids a corked stream from waiting forever to drain the stream.

Fixes #946

src/modules/module-protocol-pulse/pulse-server.c

@@ -3752,6 +3752,8 @@ static int do_drain_stream(struct client *client, uint32_t command, uint32_t tag
 
 	stream->drain_tag = tag;
 	stream->draining = true;
+	pw_stream_set_active(stream->stream, true);
+
 	return 0;
 }