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/ffa/hypervisor/cpu_cycles.c b/src/ffa/hypervisor/cpu_cycles.c
index 4496e6b..cf9f4bb 100644
--- a/src/ffa/hypervisor/cpu_cycles.c
+++ b/src/ffa/hypervisor/cpu_cycles.c
@@ -75,7 +75,7 @@
 
 	switch (func) {
 	case FFA_YIELD_32:
-		/* Fall through. */
+		[[fallthrough]];
 	case FFA_MSG_SEND_DIRECT_REQ_64:
 	case FFA_MSG_SEND_DIRECT_REQ_32:
 	case FFA_MSG_SEND_DIRECT_REQ2_64:
@@ -83,7 +83,7 @@
 		*next_state = VCPU_STATE_BLOCKED;
 		return true;
 	case FFA_MSG_WAIT_32:
-		/* Fall through. */
+		[[fallthrough]];
 	case FFA_MSG_SEND_DIRECT_RESP_64:
 	case FFA_MSG_SEND_DIRECT_RESP_32:
 	case FFA_MSG_SEND_DIRECT_RESP2_64: