MAINTAINERS/bpf: Update Andrii's entry.

MAINTAINERS/bpf: Update Andrii's entry.

Andrii has been a de-facto maintainer for libbpf and other components.
Update maintainers entry to acknowledge his work de-jure.

The folks with git write permissions will continue to follow the rule
of not applying their own patches unless absolutely trivial.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201112180340.45265-1-alexei.starovoitov@gmail.com