pdumder.c: Fix bug#68690

pdumder.c: Fix bug#68690

The problem was that the offset computed for the `charset_table` array
was a multiple of 4 but the `struct charset` needed an alignment on
a multiple of 8, so `dump_charset` inserted 4 bytes of padding,
whereas you can't have padding at the beginning of an array.

* src/pdumper.c (dump_charset): Don't set alignment here.
(dump_charset_table): Set it here instead.