Migrate to threading_helpers.h

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/tests/src/threading_helpers.c b/tests/src/threading_helpers.c
index 5fbf65b..3805934 100644
--- a/tests/src/threading_helpers.c
+++ b/tests/src/threading_helpers.c
@@ -6,6 +6,7 @@
  */
 
 #include <test/helpers.h>
+#include <test/threading_helpers.h>
 #include <test/macros.h>
 
 #if defined(MBEDTLS_TEST_MUTEX_USAGE)