QQuickItemPrivate: Define the debug counters out of line

Declare the QT_BUILD_INTERNAL counters as plain static members and
define them in qquickitem.cpp.

Inline variables get a definition in every module that uses them, so a
test module can read its own copy that libQt6Quick never updates. Define
the counters once, in libQt6Quick.

Amends 4db3146f69dba3ffa4c8d9359643815d54949413 (6.11).

Fixes: QTBUG-151455
Pick-to: 6.12
Change-Id: I8346def30ce3bf47421a969e002155d914e4854d
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>