Fixes memory leak of KItemListHeader

Summary:
Fixes memory leak of KItemListHeader
The KItemListHeader passed the listView parent object to the QObject
base class, but that pointer seems to always be nullptr causing the
KItemListHeader to not be memory managed by anything. Instead simple
use the listView as parent pointer.

Test Plan: This leak was found using Address Sanitizer

Reviewers: elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: elvisangelaccio, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D14453