Change the QtSerialPort path after the move to qt from playground.

See also: https://codereview.qt-project.org/49127



Change-Id: Ied889b16cd9a01211c7f48a0b1162f3bb8bf3692
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>
Reviewed-by: Janne Anttila <janne.anttila@digia.com>