bpf: update BPF LSM designated reviewer list

bpf: update BPF LSM designated reviewer list

Adding myself in place of both Brendan and Florent as both have since
moved on from working on the BPF LSM and will no longer be devoting
their time to maintaining the BPF LSM.

Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
Acked-by: KP Singh <kpsingh@kernel.org>
Link: https://lore.kernel.org/r/ZgMhWF_egdYF8t4D@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>