selftests/sched_ext: Include common.bpf.h to avoid build failure

selftests/sched_ext: Include common.bpf.h to avoid build failure

In scx-cid patchsets, sched_ext selftest failed to build with following
error:

non_scx_kfunc_deny.bpf.c:17:6: error: conflicting types for 'scx_bpf_kick_cpu'
17 | void scx_bpf_kick_cpu(s32 cpu, u64 flags) __ksym;
|      ^
tools/testing/selftests/sched_ext/build/include/vmlinux.h:136300:13: note: previous declaration is here
136300 | extern void scx_bpf_kick_cpu(s32 cpu, u64 flags, const struct bpf_prog_aux *aux) __weak __ksym;
|             ^
non_scx_kfunc_deny.bpf.c:26:23: error: too few arguments to function call, expected 3, have 2
26 |         scx_bpf_kick_cpu(0, 0);
|         ~~~~~~~~~~~~~~~~     ^
tools/testing/selftests/sched_ext/build/include/vmlinux.h:136300:13: note: 'scx_bpf_kick_cpu' declared here
136300 | extern void scx_bpf_kick_cpu(s32 cpu, u64 flags, const struct bpf_prog_aux *aux) __weak __ksym;

The root cause is on scx core part, but we can avoid this by including
common.bpf.h and remove scx_bpf_kick_cpu() to make it more robust, just
like the usage in other xx.bpf.c.

Link: https://lore.kernel.org/sched-ext/20260421071945.3110084-1-tj@kernel.org/
Signed-off-by: Zhao Mengmeng <zhaomengmeng@kylinos.cn>
Tested-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>