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>