selftests/sched_ext: add order-only dependency of runner.o on BPFOBJ
The runner.o may start building before libbpf headers are installed,
and as a result build fails. This happened a couple of times on
libbpf/ci test jobs:
* https://github.com/libbpf/ci/actions/runs/
11447667257/job/
31849533100
* https://github.com/theihor/libbpf-ci/actions/runs/
11445162764/job/
31841649552
Headers are installed in a recipe for $(BPFOBJ) target, and adding an
order-only dependency should ensure this doesn't happen.
Signed-off-by: Ihor Solodrai <ihor.solodrai@pm.me>
Signed-off-by: Tejun Heo <tj@kernel.org>