Migrate to threading_helpers.h

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/programs/ssl/ssl_test_lib.c b/programs/ssl/ssl_test_lib.c
index b49dd67..d562cb1 100644
--- a/programs/ssl/ssl_test_lib.c
+++ b/programs/ssl/ssl_test_lib.c
@@ -13,7 +13,7 @@
 #include "ssl_test_lib.h"
 
 #if defined(MBEDTLS_TEST_HOOKS)
-#include "test/helpers.h"
+#include "test/threading_helpers.h"
 #endif
 
 #if !defined(MBEDTLS_SSL_TEST_IMPOSSIBLE)