Fix silly typo in a macro for QAtomicInt's constructor

The T was missing in QT_BASIC_ATOMIC_HAS_CONSTRUCTORS. This makes
QAtomicInt's constructor become constexpr.

Change-Id: Ibe58ff36517c5d05ce8af9c0f28169fa63521632
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>