Include the raw STCP server API in adapter regression tests

tests/test_stcp_link.c

@@ -1,6 +1,7 @@
 // test_stcp_link.c — STCP link layer: server listen/accept (slim config) + client connect (raw stcp_client)
 #include "stcp_link.h"
 #include "stcp_client.h"
+#include "stcp_server.h"
 #include "secure_channel.h"
 #include "utun_instance.h"
 #include "etcp.h"