SPM: fix compilation warning

Fix "unused variable" compilation warning.

Change-Id: I460c4d612184ea57e47ee8da050495435c21912d
Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>
(cherry picked from commit ca9be3b)
Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>

Author: tomi-font

secure_fw/spm/core/backend_ipc.c

@@ -125,7 +125,9 @@ static void prv_process_metadata(struct partition_t *p_pt)
     struct runtime_metadata_t *p_rt_meta;
     service_fn_t *p_sfn_table;
     uint32_t allocate_size;
+#if TFM_ISOLATION_LEVEL != 1
     FIH_RET_TYPE(bool) fih_rc;
+#endif
 
     p_pt_ldi = p_pt->p_ldinf;
     p_srv_ldi = LOAD_INFO_SERVICE(p_pt_ldi);