add attributes to suppress warnings

While C23 features supersede these, these work on gcc, clang and tcc.

source/actkbd.c

@@ -12,7 +12,7 @@ static int got_hup = 0;
 
 int verbose = 0;
 
-static void hup_handler(int signum) {
+static void hup_handler(int signum __attribute__((unused))) {
 	got_hup = 0;
 }
 
@@ -34,6 +34,7 @@ int main(int argc, char *const argv[]) {
 		case 'c': config  = optarg; break;
 		case 'h':
 			ret = EXIT_SUCCESS;
+			__attribute__((fallthrough));
 		default:
 			printf("actkbd [-d DEVICE|-c CONFIG_FILE|-h|-v]\n");
 			exit(ret);