Jamcrc should be crate level public

This doesn't make any functional difference (because the module isn't
public) but whatever, still want to do it

src/crc.rs

@@ -1,7 +1,7 @@
 // SPDX-FileCopyrightText: 2023 Joshua Goins <josh@redstrate.com>
 // SPDX-License-Identifier: GPL-3.0-or-later
 
-pub struct Jamcrc {
+pub(crate) struct Jamcrc {
     table: [u32; 256],
 }