Fix DB2 driver X64 Linux Build

On X64 Linux DB2 driver build give an error on BIGINT conversion to
QVariant, casting it to qint64 solved the problem.

Task-number: QTBUG-20172
Change-Id: Id11b8a4b1f4ed3d5ed9854dfb53526d401b7d97d
(cherry picked from qtbase/c8e34ed678145f2e5709d39007e27e186d1bc1f9)
Reviewed-by: Mark Brand <mabrand@mabrand.nl>