bpf: Add MAINTAINERS entry for BPF LSM

Similar to XDP and some JITs, also add Brendan and Florent who have been
reviewing all my patches internally as reviewers. The patches are expected
as usual to go via the BPF tree(s) / list / merge workflows.

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20201125202404.1419509-1-kpsingh@chromium.org