input/chronovu_la8: address file size data type nits

input/chronovu_la8: address file size data type nits

Although the file format handled by this input module appears to be of
fixed size (8MiB plus 5 more bytes), it's more reliable to use a data
type for the file size that is larger than "an int". Although off_t
would be most portable, use uint64_t to match the code which passes the
parameter to the input module.