tools/sched_ext: Provide a compatible helper for scx_bpf_events()

Introduce __COMPAT_scx_bpf_events() to use scx_bpf_events() in a
compatible way also with kernels that don't provide this kfunc.

This also fixes the following error with scx_qmap when running on a
kernel that does not provide scx_bpf_events():

 ; scx_bpf_events(&events, sizeof(events)); @ scx_qmap.bpf.c:777
 318: (b7) r2 = 72                     ; R2_w=72 async_cb
 319: <invalid kfunc call>
 kfunc 'scx_bpf_events' is referenced but wasn't resolved

Fixes: 9865f31d ("sched_ext: Add scx_bpf_events() and scx_read_event() for BPF schedulers")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>