weston-test-runner.h includes wayland-util.h, therefore it needs
wayland-client. A partial dependency with just compile_args might have been
enough as it does not seem to use functions from wayland-util.c, but safer this
way and no harm.
Fixes: https://lists.freedesktop.org/archives/wayland-devel/2020-January/041149.html
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>