refactor: warn against implicit fallthrough

Enable the `-Wimplicit-fallthrough` warning, which will warn if `case`
statements in a `switch` block fall through to the next `case`.
Intentional fallthrough can be signalled with the `[[fallthrough]]`
attribute which was added in C23.

Change-Id: I421cacadafba6457847c3a067483c97fdd9896cf
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/src/api.c b/src/api.c
index 5fb4daf..ebb6561 100644
--- a/src/api.c
+++ b/src/api.c
@@ -3245,7 +3245,7 @@
 		    0U) {
 			return false;
 		}
-		/* Intentional fall-through */
+		[[fallthrough]];
 	case FFA_MEM_LEND_64:
 	case FFA_MEM_LEND_32:
 	case FFA_MEM_DONATE_64: