bpf, lsm: Make BPF_LSM depend on BPF_EVENTS

LSM and tracing programs share their helpers with bpf_tracing_func_proto
which is only defined (in bpf_trace.c) when BPF_EVENTS is enabled.

Instead of adding __weak symbol, make BPF_LSM depend on BPF_EVENTS so
that both tracing and LSM programs can actually share helpers.

Fixes: fc611f47 ("bpf: Introduce BPF_PROG_TYPE_LSM")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200330204059.13024-1-kpsingh@chromium.org