Build: Fix IAR build error

Following ns_agent_tz introduction build is failing with IAR 8.5.1

Change-Id: I01061731ddb28e4e9e33a5f12e31c9a3a612aad7
Signed-off-by: Michel Jaouen <michel.jaouen@st.com>
diff --git a/secure_fw/partitions/ns_agent_tz/ns_agent_tz.c b/secure_fw/partitions/ns_agent_tz/ns_agent_tz.c
index 3a1e713..d140831 100644
--- a/secure_fw/partitions/ns_agent_tz/ns_agent_tz.c
+++ b/secure_fw/partitions/ns_agent_tz/ns_agent_tz.c
@@ -11,6 +11,7 @@
 #include "tfm_hal_platform.h"
 
 #if defined(__ICCARM__)
+static uint32_t ns_agent_tz_init_c(void);
 #pragma required = ns_agent_tz_init_c
 #endif