Link Qt X11Extras in kwin core library

QX11Info is used in `platform.cpp`, which is used by both
kwin_x11 and kwin_wayland. The latter only gets it implicitly
through KGlobalAccelPrivate.

Ideally, platform.cpp was split to not require QX11Info in
kwin_wayland in the first place.

Signed-off-by: Victoria Fischer <victoria.fischer@mbition.io>