[helpers/killer] Force to platform XCB

Summary:
The killer can only kill X11 windows. It is also only invoked for Client
and not for ShellClient. The tool interacts with QX11Info which means it
would crash if tried to use on Wayland.

Thus force platform xcb on the tool.

Test Plan: Killer shows for Xwayland windows.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D3228