QDateTime: fix unused function warning on non-Windows

Change-Id: I8fbf1f8b0ebfdf58cd72c724326227dc1610e746
Reviewed-by: John Layt <jlayt@kde.org>

src/corelib/tools/qdatetime.cpp

@@ -3228,12 +3228,12 @@ bool QDateTime::operator<(const QDateTime &other) const
     \sa currentDateTime(), currentDateTimeUtc(), toTime_t(), toTimeSpec()
 */
 
+#if defined(Q_OS_WIN)
 static inline uint msecsFromDecomposed(int hour, int minute, int sec, int msec = 0)
 {
     return MSECS_PER_HOUR * hour + MSECS_PER_MIN * minute + 1000 * sec + msec;
 }
 
-#if defined(Q_OS_WIN)
 QDate QDate::currentDate()
 {
     QDate d;