QTest: add missing <chrono> to qtestsupport_core.h

Don't rely on transitive includes.

Pick-to: 6.7
Change-Id: I350922a47842ad5bdad0dc3f8349b0c82dd4bd0d
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/kernel/qtestsupport_core.h

@@ -7,6 +7,8 @@
 #include <QtCore/qcoreapplication.h>
 #include <QtCore/qdeadlinetimer.h>
 
+#include <chrono>
+
 QT_BEGIN_NAMESPACE
 
 namespace QTest {