lightningd: fix format specifier for bitcoin_tx_weight in splice log.

bitcoin_tx_weight() returns size_t, so use %zu instead of %lu to avoid
a -Wformat error under -Werror on platforms where size_t != unsigned long.

Changelog-Fixed: Builds on linux/amd64 to push to Dockerhub

Author: sangbida

lightningd/channel_control.c

@@ -621,7 +621,7 @@ static void send_splice_tx(struct channel *channel,
 	u8* tx_bytes = linearize_tx(tmpctx, tx);
 
 	log_debug(channel->log,
-		  "Broadcasting splice tx %s for channel %s. Final weight %lu",
+		  "Broadcasting splice tx %s for channel %s. Final weight %zu",
 		  tal_hex(tmpctx, tx_bytes),
 		  fmt_channel_id(tmpctx, &channel->cid),
 		  bitcoin_tx_weight(tx));