libpayload: Fix clang fallthrough warning

Clang does not recognize comments to indicate falltrough is intended
behavior.

Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Change-Id: Idcf7a24fc763b80863902702172b4ea950e132b8
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77431
Reviewed-by: Martin L Roth <gaumless@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>

payloads/libpayload/drivers/i8042/keyboard.c

@@ -578,7 +578,7 @@ int keyboard_getchar(void)
 				/* vulcan nerve pinch */
 				if ((modifier & KB_MOD_ALT) && reset_handler)
 					reset_handler();
-				/* fallthrough */
+				__fallthrough;
 			default:
 				ret = 0;
 			}