Use qMod(, 7) rather than % 7 in day-of-week calculation

Otherwise, for the first day of a negative year, we'd get an invalid
day of the week (required to be in the range 1 through 7).

This is a follow-up to commit 1f4b237dad.

Change-Id: If1afcd42065c26d5fa5799b0cd47921a3d424dc8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/time/qgregoriancalendar.cpp

@@ -144,7 +144,7 @@ int QGregorianCalendar::yearStartWeekDay(int year)
 {
     // Equivalent to weekDayOfJulian(julianForParts({year, 1, 1})
     const int y = year - (year < 0 ? 800 : 801);
-    return (y + qDiv(y, 4) - qDiv(y, 100) + qDiv(y, 400)) % 7 + 1;
+    return qMod(y + qDiv(y, 4) - qDiv(y, 100) + qDiv(y, 400), 7) + 1;
 }
 
 QCalendar::YearMonthDay QGregorianCalendar::julianDayToDate(qint64 jd) const