pio: add missing return statement

main() should return 0 if execution was successful.
Signed-off-by: Andre Przywara <osp@andrep.de>

pio.c

@@ -427,4 +427,6 @@ int main(int argc, char **argv)
 			exit(1);
 		}
 	}
+
+	return 0;
 }