BUILD: tcpcheck: use __fallthrough in check_proxy_tcpcheck()
This avoids 1 build warning when preprocessing happens before compiling
with gcc >= 7.
diff --git a/src/tcpcheck.c b/src/tcpcheck.c
index 366a8d0..bfe3a2c 100644
--- a/src/tcpcheck.c
+++ b/src/tcpcheck.c
@@ -3757,7 +3757,7 @@
next = get_next_tcpcheck_rule(&px->tcpcheck_rules, chk);
if (next && next->action == TCPCHK_ACT_SEND)
chk->connect.options |= TCPCHK_OPT_HAS_DATA;
- /* fall through */
+ __fallthrough;
case TCPCHK_ACT_ACTION_KW:
ha_free(&comment);
break;