STM32U5: fix unused variable

fix build warning.

Change-Id: I7d517148604fb109ddc7f437545188d60572dcd4
Signed-off-by: Ahmad EL JOUAID <ahmad.eljouaid@st.com>

(cherry picked from commit 7350b3d6a91d8ee7bf0779960d06d502aa5122d5)

Author: ahmadstm

platform/ext/target/stm/common/stm32u5xx/secure/tfm_hal_isolation.c

@@ -361,8 +361,8 @@ FIH_RET_TYPE(enum tfm_hal_status_t) tfm_hal_bind_boundary(
     bool ns_agent;
     uint32_t partition_attrs = 0;
     const struct asset_desc_t *p_asset;
-    struct platform_data_t *plat_data_ptr;
 #if TFM_ISOLATION_LEVEL == 2
+    struct platform_data_t *plat_data_ptr;
     struct mpu_armv8m_region_cfg_t localcfg;
 #endif