Ensure mutex test mutex gets free'd

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 6e0c615..b49dd67 100644
--- a/programs/ssl/ssl_test_lib.c
+++ b/programs/ssl/ssl_test_lib.c
@@ -435,6 +435,9 @@
 
 void test_hooks_free(void)
 {
+#if defined(MBEDTLS_TEST_MUTEX_USAGE)
+    mbedtls_test_mutex_usage_end();
+#endif
 }
 
 #endif /* MBEDTLS_TEST_HOOKS */