QDateTime: specify a class template argument

Although the code is legal in C++20 (due to the new CTAD rules for
alias templates), clang-cl errors out on it. Just specify the template
parameter, which we know anyways.

Change-Id: Ifdea31853d4281601cf40ed8adeb14e493a5ce04
Pick-to: 6.4
Fixes: QTBUG-107043
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/time/qdatetime.h

@@ -458,7 +458,7 @@ public:
     std::chrono::sys_time<std::chrono::milliseconds> toStdSysMilliseconds() const
     {
         const std::chrono::milliseconds duration(toMSecsSinceEpoch());
-        return std::chrono::sys_time(duration);
+        return std::chrono::sys_time<std::chrono::milliseconds>(duration);
     }
 
     QT_POST_CXX17_API_IN_EXPORTED_CLASS