sgm: Fix bl2 sources

The weak version of plat_get_mbedtls_heap() was being used.

Change-Id: I6da331a098dd1af5bb64729d5b914cfb74b8869e
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>

plat/arm/css/sgm/sgm-common.mk

 #
-# Copyright (c) 2018, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2018-2019, ARM Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -33,7 +33,8 @@ BL1_SOURCES		+=	$(SGM_CPU_SOURCES)			\
 				${CSS_SGM_BASE}/sgm_bl1_setup.c		\
 				${CSS_SGM_BASE}/sgm_plat_config.c
 
-BL2_SOURCES		+=	${SECURITY_SOURCES}
+BL2_SOURCES		+=	${SECURITY_SOURCES}			\
+				${CSS_SGM_BASE}/sgm_plat_config.c
 
 BL2U_SOURCES		+=	${SECURITY_SOURCES}