HPCC-12512 Fix incorrect parameter type on rtlWriteInt8

* This would cause the values to be truncated to 4 bytes.

Thankfully the function isn't called much.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6603 from ghalliday/issue12512a

HPCC-12512 Fix incorrect parameter type on rtlWriteInt8

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>