build: fix compilation on i386

```
plugins/keysend.c:136:47: error: format specifies type 'unsigned long' but the argument has type 'time_t' (aka 'int') [-Werror,-Wformat]
        ki->label = tal_fmt(ki, "keysend-%lu.%09lu", now.ts.tv_sec, now.ts.tv_nsec);
                                         ~~~         ^~~~~~~~~~~~~
                                         %d
```

Changelog-None

plugins/keysend.c

@@ -134,7 +134,7 @@ static struct command_result *htlc_accepted_call(struct command *cmd,
 
 	ki = tal(cmd, struct keysend_in);
 	memcpy(&ki->payment_preimage, preimage_field->value, 32);
-	ki->label = tal_fmt(ki, "keysend-%lu.%09lu", now.ts.tv_sec, now.ts.tv_nsec);
+	ki->label = tal_fmt(ki, "keysend-%lu.%09lu", (unsigned long)now.ts.tv_sec, now.ts.tv_nsec);
 	ki->payload = tal_steal(ki, payload);
 	ki->preimage_field = preimage_field;