Use QStringList::join(QChar) overload where applicable [qmake]

This is an automated change performing the following replacements:
                    join\("(.)"\) -> join('\1')
  join\(QLatin1String\("(.)"\)\)  -> join(QLatin1Char('\1'))
  join\(QStringLiteral\("(.)"\)\) -> join(QLatin1Char('\1'))

Change-Id: I9c9964703dedfdab6e7bfac80be22bd5570e2e49
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
(cherry picked from qtbase/f4d47945ba17db276e94046473816014ed0342e9)
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>