Use sal_Int32 to satisfy oox/helper/helper.hxx's convertLittleEndian

Fixes build error "no matching function for call to oox::ByteOrderConverter::convertLittleEndian(int&)"

Change-Id: I899f151ff99737247ce4090f59897eba842c40ba
Reviewed-on: https://gerrit.libreoffice.org/7099
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>