BUG/MINOR: config: fix leak on proxy.conn_src.bind_hdr_name

Leak for parsing of option usesrc of the source keyword.

This can be backported to 1.8.
diff --git a/src/haproxy.c b/src/haproxy.c
index e36e020..cdbc3dc 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2503,6 +2503,9 @@
 		free(p->rdp_cookie_name);
 		free(p->invalid_rep);
 		free(p->invalid_req);
+#if defined(CONFIG_HAP_TRANSPARENT)
+		free(p->conn_src.bind_hdr_name);
+#endif
 		if (p->conf.logformat_string != default_http_log_format &&
 		    p->conf.logformat_string != default_tcp_log_format &&
 		    p->conf.logformat_string != clf_http_log_format)