MINOR: log: Don't systematically set LW_REQ when a sample expr is added

When a log-format string is parsed, if a sample fetch is found, the flag LW_REQ
is systematically added on the proxy. Unfortunately, this produce a warning
during HAProxy start-up when a log-format string is used for a tcp-check send
rule. Now this flag is only added if the parsed sample fetch depends on HTTP
information.
diff --git a/src/log.c b/src/log.c
index 4f3c7c7..bacc04e 100644
--- a/src/log.c
+++ b/src/log.c
@@ -532,7 +532,8 @@
 	 * now on, but this will leave with sample capabilities soon.
 	 */
 	curpx->to_log |= LW_XPRT;
-	curpx->to_log |= LW_REQ;
+	if (curpx->http_needed)
+		curpx->to_log |= LW_REQ;
 	LIST_ADDQ(list_format, &node->list);
 	return 1;