qt5webkit: get source code from Qt5 unofficial builds

qt5webkit: get source code from Qt5 unofficial builds

Instead of messing with git we can get tarballs of obsolete but still
existing Qt5 submodules as we used to but from a bit different
location.

It turned out Qt people still create packages for obsolete submodules
for so-called "unofficial builds", see [1].

[1] https://wiki.qt.io/Qt-5-unofficial-builds

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Seiderer <ps.report@gmx.net>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Jerzy Grzegorek <jerzy.grzegorek@trzebnica.net>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Julien Corjon <corjon.j@ecagroup.com>
Cc: Gary Bisson <gary.bisson@boundarydevices.com>
Cc: Jérôme Pouiller <jezz@sysmic.org>
Reviewed-by: Julien Corjon <corjon.j@ecagroup.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>