Fix build without X11/XCB

Summary:
It's missing the include for QQuickWindow in that case because of
the incorrect nesting and fails with "error: invalid use of
incomplete type class QQuickWindow".

Test Plan:
Tested with -DCMAKE_DISABLE_FIND_PACKAGE_XCB=TRUE/FALSE
and -DUSE_KDE4:BOOL=TRUE/FALSE.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9385