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 <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

tools/testing/selftests/sched_ext/Makefile

···
 
 testcase-targets := $(addsuffix .o,$(addprefix $(SCXOBJ_DIR)/,$(auto-test-targets)))
 
-$(SCXOBJ_DIR)/runner.o: runner.c | $(SCXOBJ_DIR)
+$(SCXOBJ_DIR)/runner.o: runner.c | $(SCXOBJ_DIR) $(BPFOBJ)
 	$(CC) $(CFLAGS) -c $< -o $@
 
 # Create all of the test targets object files, whose testcase objects will be