Dualcpu: Align with TF-M to build mailbox RTOS specific file

Include NS mailbox RTOS specific file in NS interface build in
dual-cpu scenario.

Signed-off-by: David Hu <david.hu@arm.com>
Change-Id: I33471a86b155a88473c6b00427cd9a61097135a0
diff --git a/app/CMakeLists.txt b/app/CMakeLists.txt
index c260b7d..8822fea 100644
--- a/app/CMakeLists.txt
+++ b/app/CMakeLists.txt
@@ -1,5 +1,5 @@
 #-------------------------------------------------------------------------------
-# Copyright (c) 2020, Arm Limited. All rights reserved.
+# Copyright (c) 2020-2021, Arm Limited. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -79,8 +79,9 @@
     if (TFM_MULTI_CORE_TOPOLOGY)
         target_sources(tfm_api_ns PRIVATE
             ${INTERFACE_SRC_DIR}/tfm_ns_mailbox.c
-            ${INTERFACE_SRC_DIR}/tfm_multi_core_api.c
+            ${INTERFACE_SRC_DIR}/tfm_multi_core_ns_api.c
             ${INTERFACE_SRC_DIR}/tfm_multi_core_psa_ns_api.c
+            ${INTERFACE_SRC_DIR}/tfm_ns_mailbox_rtos_api.c
         )
     else()
         target_sources(tfm_api_ns PRIVATE