Port away from qt_qhash_seed which is going away in Qt 5.9

https://codereview.qt-project.org/188474

qSetGlobalQHashSeed exists since Qt 5.6, but this project seems to
require only 5.2, so I added a Qt version ifdef.

(cherry picked from commit 52cb4d3a7dc7383d8ca2a071debde74a392215bc)