refactor: move `inc/hf/arch/plat/ffa` to `inc/hf/ffa`

Change-Id: I05c2cd1aa3753cd60021015c4582831eb03db3c9
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/inc/hf/assert.h b/inc/hf/assert.h
index d7d7770..8e05659 100644
--- a/inc/hf/assert.h
+++ b/inc/hf/assert.h
@@ -10,6 +10,7 @@
 
 #if !defined(__cplusplus)
 
+#include <stdbool.h>
 #include <stdint.h>
 
 #include "hf/dlog.h"