libpng: Add Windows Phone to existing WinRT define

Treat Windows Phone as WinRT.

Change-Id: I74e45a199629df3efaafa6acb05f991044f5c884
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Fix QT_POINTER_SIZE on WinRT x64

Q_OS_WIN64 is not defined for WinRT, so make sure the pointer size is
correct.

Change-Id: I5a55bfd7edbfd23e0eab50fa31a76faa9e383a8d
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>