Broaden condition on use of MS's localtime_s()

It's available on MinGW, so not limited to MSVC; and the MinGW
localtime() and localtime_r() fail in some cases where it works.

Use the generic name rather than _localtime64_s(), since localtime_s()
now just calls it; and, in any case, we were assuming time_t is
__time64_t when calling it.

Change-Id: I316cc5b1a3e19cd6725555042dfaba3124a25a03
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/time/qdatetime.cpp

@@ -2619,8 +2619,8 @@ static bool qt_localtime(qint64 msecsSinceEpoch, QDate *localDate, QTime *localT
             Q_ASSERT(res == &local);
             valid = true;
         }
-#elif defined(Q_CC_MSVC)
-        if (!_localtime64_s(&local, &secsSinceEpoch))
+#elif defined(Q_OS_WIN)
+        if (!localtime_s(&local, &secsSinceEpoch))
             valid = true;
 #else
         // Returns shared static data which may be overwritten at any time