bpf: mark bpf_event_notify and bpf_event_init as static

Both of them are not declared in the headers and not used outside
of bpf_trace.c file.

Fixes: a38d1107 ("bpf: support raw tracepoints in modules")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>