bpf: export bpf_event_output()

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: NJakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: NQuentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: NJiong Wang <jiong.wang@netronome.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

kernel/bpf/core.c

@@ -1719,6 +1719,7 @@ bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
 {
 	return -ENOTSUPP;
 }
+EXPORT_SYMBOL_GPL(bpf_event_output);
 
 /* Always built-in helper functions. */
 const struct bpf_func_proto bpf_tail_call_proto = {