[tests] build fix

diff --git a/tests/gsm0408/gsm0408_test.c b/tests/gsm0408/gsm0408_test.c
index 0fe1310..cda28b1 100644
--- a/tests/gsm0408/gsm0408_test.c
+++ b/tests/gsm0408/gsm0408_test.c
@@ -74,6 +74,7 @@
 void del_timer(void) {}
 void subscr_get_by_tmsi(void) {}
 void subscr_update(void) {}
+void subscr_put(void) {}
 void db_subscriber_assoc_imei(void) {}
 void db_subscriber_alloc_tmsi(void) {}
 void db_create_subscriber(void) {}