gossipd: make seeker uintmap visible to memleak code.

```
gossipd: MEMLEAK: 0x55e22ac0f8e8
gossipd:   label=gossipd/seeker.c:621:u8
gossipd:   alloc:
gossipd:     ccan/ccan/tal/tal.c:477 (tal_alloc_)
gossipd:     gossipd/seeker.c:621 (check_timestamps)
gossipd:     gossipd/seeker.c:645 (process_scid_probe)
gossipd:     gossipd/queries.c:849 (handle_reply_channel_range)
gossipd:     gossipd/gossipd.c:436 (handle_recv_gossip)
gossipd:     gossipd/gossipd.c:509 (connectd_req)
gossipd:     common/daemon_conn.c:35 (handle_read)
gossipd:     ccan/ccan/io/io.c:59 (next_plan)
gossipd:     ccan/ccan/io/io.c:407 (do_plan)
gossipd:     ccan/ccan/io/io.c:417 (io_ready)
gossipd:     ccan/ccan/io/poll.c:453 (io_loop)
gossipd:     gossipd/gossipd.c:950 (main)
gossipd:   parents:
gossipd:     gossipd/seeker.c:132:struct seeker
```