BUILD: extcheck: needs to include stream-t.h

It uses the SF_ERR_* error codes and currently gets them via
intermediary includes.
diff --git a/src/extcheck.c b/src/extcheck.c
index aa342fc..e621918 100644
--- a/src/extcheck.c
+++ b/src/extcheck.c
@@ -36,6 +36,7 @@
 #include <haproxy/proxy.h>
 #include <haproxy/server.h>
 #include <haproxy/signal.h>
+#include <haproxy/stream-t.h>
 #include <haproxy/task.h>
 #include <haproxy/thread.h>
 #include <haproxy/time.h>