Move back timing.c to mbedtls

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index c6ee8eb..a32b4bc 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -30,6 +30,7 @@
     ssl_tls13_server.c
     ssl_tls13_client.c
     ssl_tls13_generic.c
+    timing.c
     version.c
     version_features.c
 )