perf/bpf: Remove #ifdef CONFIG_BPF_SYSCALL from struct perf_event members

This will allow __perf_event_overflow() (which is independent of
CONFIG_BPF_SYSCALL) to use struct perf_event's prog to decide whether to
call bpf_overflow_handler().

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Kyle Huey <khuey@kylehuey.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240412015019.7060-4-khuey@kylehuey.com