Migrate to threading_helpers.h

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/programs/test/metatest.c b/programs/test/metatest.c
index b8dffa9..82ecf95 100644
--- a/programs/test/metatest.c
+++ b/programs/test/metatest.c
@@ -31,6 +31,7 @@
 #include <mbedtls/platform.h>
 #include <mbedtls/platform_util.h>
 #include "test/helpers.h"
+#include "test/threading_helpers.h"
 #include "test/macros.h"
 
 #include <stdio.h>