Build fix for 10.6

Applied patch pointed to from QTBUG-31363 that ensures that
we include system headers before Qt/project headers.

Task-number: QTBUG-31363
Change-Id: I1aae10c9be0337aab8f84044e2fcfe04e1e75c41
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>