bpf_event_output() is useful for offloads to add events to BPF
event rings, export it. Note that export is placed near the stub
since tracing is optional and kernel/bpf/core.c is always going
to be built.
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com> Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com> Reviewed-by: Jiong Wang <jiong.wang@netronome.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>