QString::vasprintf(): Use quintptr when casting pointer for %p.

Previously, the macro Q_OS_WIN64 was checked, causing warnings:

tools\qstring.cpp(6183): warning C4311: 'reinterpret_cast': pointer truncation from 'void *' to 'unsigned long'
tools\qstring.cpp(6183): warning C4302: 'reinterpret_cast': truncation from 'void *' to 'unsigned long'

when compiling WinRT/64bit, where it is not defined.

Change-Id: Ib9d8405108c85170aba18b13f9c64083136bc5ee
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@theqtcompany.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>