qxptype_traits.h: include q20type_traits.h

A qNNfoo.h should always include qMMfoo.h, MM=NN-3, if that exists, to
avoid a user having to include both qNNfoo.h and qMMfoo.h, which later
become two <foo> includes on automated replacement. For the purpoes of
this argument, NN=xp is considered equal to max({MM : ∃qMMfoo.h}) + 3.

Following this logic, qxptype_traits.h should include q20type_traits.h
(q23type_traits.h doesn't, yet, exist).

It didn't, so fix.

Amends def2a3de37.

Pick-to: 6.7 6.5
Change-Id: I8153d241783dc4e505fddf2407d27990e5c522f4
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 720ae32f79038a5db763a2371c0e8d25d32d93cf)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/global/qxptype_traits.h

@@ -8,7 +8,7 @@
 #include <QtCore/qtconfigmacros.h>
 #include <QtCore/qcompilerdetection.h>
 
-#include <type_traits>
+#include <QtCore/q20type_traits.h>
 
 //
 //  W A R N I N G